Thursday, November 13, 2014

Enphase and SolarEdge

The most popular microinverter and the most popular DC optimizer butt heads a lot in the current solar market.  Both are panel-level MPP trackers that maximize individual panel yield.  Both make the array more resistant to shading and panel mismatch.  And both are fully compliant with the latest Rapid Shutdown requirements for solar systems.

So, what should you use, savvy solar installer?

I can't speak for financials personally, but from a pure technical standpoint I know what I would put on my roof, and it's all AC baby!  Enphase would be my choice for a residential system, and I base that decision on ease of install, system reliability, and annual energy production.

Installing Enphase means mounting microinverters to your solar racking system and running AC trunk cable to a junction box where it transitions into a raceway.  The trunk cable is outdoor rated and extremely robust, and comes included with every Enphase system.  Solaredge's procedure is similar, but there is one tradeoff.  While they don't run an AC trunk cable at the array, they still have to use a junction box to transition into a raceway.  Additionally, a SolarEdge system has to use a string inverter to manage the optimizers and create AC power, since they only do a DC-DC power conversion.  Without a string inverter to mount, Enphase has a clear advantage over SolarEdge on ease of install in residential systems.

System reliability is hard to compare without having long-term hands-on experience with both systems.  When you are mounting a small power electronics box under each panel for each system, it's a wash, and each system features a 25 year warranty.  However, there is one key difference between Enphase and Solaredge that makes Enphase more reliable, and that is the single point of failure in the SolarEdge string inverter that is required on every system.  Enphase doesn't have a single failure point in it's system, but with SolarEdge if the inverter fails the entire system goes down.  Additionally, the SolarEdge string inverter only has a 12 year warranty, which points to it failing more often than microinverters or optimizers.

Annual energy production is the easiest thing to compare, especially if actual site data is available. Thankfully, we have such data in a study by PV Evolution Labs, a 3rd party testing agency in California.  The study concludes that Enphase systems produced at least 2.4% more energy on a kWH per installed kWP basis.  To confirm their results, I created a PVSyst simulation to compare the two systems (available upon request).  The difference in production between the two systems? 2.4%!  In searching for alternative test data, I found an older test by SolarEdge that compared it to SMA and Enphase under shaded conditions, but the results mainly focus on how SolarEdge outperforms SMA, not Enphase which is almost identical in their results.

I'm doing my homework and I keep coming up with Enphase microinverters as the superior panel-level MPPT solution, especially for residential systems.  The SolarEdge system is not without its benefits, but it's single failure point and added labor on string inverter installation make Enphase the optimal choice.

Friday, March 21, 2014

Solar Array Commissioning –Production Analysis

When you install a system, sometimes you are asked to verify that it is producing the expected amount of power.  Simply reading the inverter LCD screen will give you the AC power production, but that information hasn’t been checked against the site conditions so you have no way of knowing if it’s correct or not.  To prove it using math, you have to perform a production analysis.  I am going to go over the tools and methods to analyze a solar array for power production based on temperature and solar irradiance.

This method uses an IR thermometer to measure solar panel temperature and a solar pyranometer to measure solar irradiance.  IR thermometers are fairly common and can be found online or in local hardware stores.
Figure 1: Handheld IR Thermometer

To properly use the IR thermometer, “aim” it at the back of a solar panel from 3 – 6 feet away and note the temperature. There will be a laser dot to show where you are measuring temperature and it should be in the middle of the solar panel.

Solar pyranometers are a little more specialized and usually have to be purchased online.  In shopping for a solar pyranometer, you want a handheld unit you can bring on a roof with you.  I prefer to use the Daystar Solar Meter for its simplicity and portability (

Figure 2: Handheld Solar Pyranometer measuring irradiance on the array plane

To properly use the solar pyranometer, the meter’s collector cell has to be in the same plane as the solar panel to measure the solar irradiance incident on the solar panel plane (as shown in Figure 2). 

Knowing how to collect the data, the equation for continuous solar array production is:

Actual Power = (Power @ STC)*(Irradiance Factor)*(Temperature Factor)*(System Efficiency)
  • Power @ STC is the rated DC power of the solar array.
  • Irradiance Factor is the quotient of measured solar irradiance divided by 1000 W/m2 (STC)
  • Temperature Factor uses the measured cell temperature of the solar panel and the temperature coefficient of power from the solar panel data sheet to find the temperature derate factor.
  • System Efficiency is the efficiency of the DC wiring, inverter efficiency, and other factors.

The irradiance factor is determined from measuring the solar irradiance in the plane of the solar array (orientation is everything!).  Dividing by 1000 W/m2 converts it into a decimal factor that is applied to the overall power of the solar array.  This value is typically less than 1 but can be greater than 1 if you live in a location that gets excellent solar irradiance or take the measurement when there are optimal production conditions.

The temperature factor uses the same equation that is used to calculate maximum voltage in strings of solar panels.  The only difference is the maximum power coefficient (TCP, found on the solar panel data sheet) is used instead of open circuit voltage. That equation is:

Temperature Factor = 1 + (TCP*(Cell Temperature - TSTC))

Temperature units are always in Celsius, and TSTC = 25o C.  The cell temperature is the value measured using the IR thermometer on the back of a solar panel.  The temperature factor should always be less than 1 unless you take measurements when the temperature is very low.  In full sun, most solar panels will be well above 100o F (~38o C).

System efficiency is an aggregate efficiency that includes several factors that affect the production of the solar array.  Wiring losses and inverter efficiency are the most visible, but some others include (with their typical values):

Solar Panel Mismatch
 0.97 (string inverters) or 1.0 (microinverters)
Solar Panel Soiling
1.0 (new systems), 0.99 (dusty), 0.98 or less (dirty)
Solar Panel nameplate tolerance
1.0 (positive-only), 0.99 (pos/neg)
1.0 (new systems), 0.995 (1 yr), 0.99 (2 yr), etc
Wiring Losses
0.98 (DC wiring + connection losses)
Inverter Efficiency
0.96 (CEC efficiency from inverter datasheet)

For a new system production analysis using positive-tolerance solar panels with a string inverter, the overall system efficiency will be around 0.9.  For a new microinverter system the efficiency will be approximately 0.93.

Going back to the array production equation, it can be seen how the various factors work to reduce total solar array output from STC rating to real-world performance.  The largest influence is higher temperatures having a noticeable and direct effect on the power production of the solar panels.  To mitigate this, the solar array can be mounted in a way to receive more ventilation (spaced off roof, no obstructions) and overall power production will improve as a result.

Tuesday, February 25, 2014

Selling Value vs. Price - The Eternal Struggle

The mark of a good salesman is identifying customer needs and providing them the products they need to fulfill those needs.  I’m taking a break from design posts to take a look at the sales side of solar and some of its unique challenges.  I've enlisted the expertise of our resident sales guru, Tim McGivern.  Tim is a seasoned veteran of sales with a firm grasp of customer interaction, and he comes with a wealth of knowledge and experience.  Today’s challenge is how to sell quality and value versus pure cost in a solar project.

Find the right solution for your target customers and the products you use, and focusing like a laser on them to efficiently and effectively grow your business while leaving your customers satisfied.  In dealing with your customers, it is your job to make that value proposition and demonstrate the unique quality of your products.  The pitfall in the value vs. price relationship is when you try to sell someone purely on the price of a solar system.  This is a battle that you will never win in the long run.  If your customer is a price shopper, they will have zero loyalty to you and fail to appreciate your integrity and good faith in delivering them a quality product.  If you get the sale, you are left with low margins and no room for error in the installation.  If you don’t, it’s more time spent on someone that’s just going to shop your quote to competitors.  It doesn't make sense to do price-driven sales when you are using high-quality solar equipment. Nor does it make sense to install low quality equipment that will cost you future labor in repairs and stressful customer service.

The value inherent in tier one solar panels, inverters, and racking cannot be overstated.  It can be the difference of all solar cells being lined up on mono-crystalline panels, a sturdy module frame, or higher efficiency cells.  A company like Hyundai offers quite a lot in added value because they are a diversified company that makes an efficient solar panel and isn’t affected by potential tariff issues.  Canadian Solar, while made in China, has unique value that differentiates it from other Chinese modules like Trina or Yingli, for example a high PTC/STC production ratio which points to excellent real-world performance.  While Power One doesn't have the track record Enphase does with Microinverters, they have unique value in being a bankable company that you can have confidence will be around for years to come.  Unirac combines excellent pricing with extremely robust engineering and long-term performance that no competitor can match.  These are just a few examples.

Finding the unique value in the products you use is the best way to compete against cheap competitors that sell lower quality solar equipment.  Let it shine!

Friday, February 14, 2014

Grid Tied Inverter Overloading Analysis (with PVSyst)

For today’s post, I brought out the solar modeling software and gotten myself into trouble (grid tied inverter loading analysis).  System analysis and solar modeling software is a delicate subject, there are dozens of variables and different models can produce different results.  I've tried to keep things as simple as possible for the purpose of this analysis.

When you hear about inverter loading/overloading, it refers to the ratio of Solar DC to Inverter AC power (Watts) that you design into your system.  This can be expressed as a percentage value.  For example, a system that has an inverter that’s “20% overloaded” (or 120% loaded) would mean the DC array size is 20% larger than the AC rating of the inverter.  A system that is 0% overloaded (or 100% loaded) would have a solar array and inverter that equal each other in size.  For the purpose of this analysis, I created three different inverter loading scenarios using Power One string inverters and Canadian Solar panels.

Solar Array
Inverter Load
Power One 6000   (6 kW)
(24) Canadian Solar 250W Poly
2 Strings of 12
Power One 5000   (5 kW)
Power One 4.2     (4.2 kW)

It’s important to keep the string size consistent when making comparisons like this to cut down on loose variables.  In this case, I was lucky to find a situation where an identical solar array can be compared across three different inverter loading scenarios in a fairly linear fashion.  The only difference between the inverters is a slight decrease in the CEC efficiency (0.5%) from the 5 & 6kW inverters to the 4.2kW version, while maximum efficiency and voltage windows are almost uniform across all three.  I corrected for the CEC efficiency loss by increasing the 4.2kW inverter production numbers by 0.5%, but in the end this had little to no effect on the data.

The different loading ratios have been simulated across four different cities for average annual production:

Albuquerque, NM – A good representation of a dry climate with high-intensity sun.

Miami, FL – A sunny site with interesting weather effects and a lot of humidity.

Chicago, IL – A colder site that gets less sunshine with long cloudy winters

Kansas City, MO – An average site that has characteristics of the other three sites.

To estimate production, I’ve used PVSyst (V5.65) for the simulation software.  The system orientation was set at a 30 degree tilt and true south azimuth.  The following results were obtained for average annual production across all sites and loading conditions:

Inverter Size:
% Loss
% Loss
Albuquerque, NM
Miami, FL
Chicago, IL
Kansas City, MO

Production numbers are to be expected for the four cities.  Some may be surprised that Miami is so low while Kansas City is higher.  The reason is Miami has a lot of humidity and a fairly uniform temperature profile year-round.  Kansas City has hot summers, but their winters are fairly cold and that boosts solar production, and their dryer climate helps more of the sun’s rays hit the solar array.  Chicago has the worst solar production as expected, but they are affected more by inverter loading than Miami.  This is due to colder overall temperatures allowing the solar array to produce more continuous power during optimal conditions, which can cause clipping on an overloaded inverter. 

What jumps out immediately is the change in production in going from a 6kW inverter to a 5kW inverter, or lack thereof.  There is virtually zero difference in production for three of the sites by going to an inverter loaded 120%, and for Albuquerque it’s only half a percentage point off the top.  This is negligible, and after the first year or two of service there will be no difference due to degradation of the solar panels (which typically lose ~0.5% of their production per year installed).  It can be seen that overloading the inverter by 20% is something that should be designed into any solar job, while the benefit from matching the inverter rating with the solar array size just isn't there.

The 140% loaded inverter has more significant production losses than the 120% inverter.  For Albuquerque especially, the difference is too large to be ignored, and overloading the inverter that much should not be attempted at similar sites for optimal orientations.  For the other three cities, the power loss is pretty bad but there is still hope.  The simulation parameters are for a system tilted at 30 degrees, oriented optimally with respect to the sun, all with zero shading.  If a system is being considered with sub optimal orientation or shading, anything that might adversely affect production, an inverter loaded to 140% isn't out of reach and in many cases will work just as well.  The production difference is only 0.8% for Miami, so temperate sites without cold winters could have 40% overloaded inverters without any trouble.

What are the biggest takeaways from this?  The better the site is for solar (days of sunshine, low humidity, cold winters), the less the inverter should be overloaded.  The minimum for inverter loading falls around 120% for the United States.  Anything lower and you are wasting inverter capacity, anything higher and you need to consider the site conditions before proceeding.  Temperate climates where winter temperatures do not drop below zero have the most options with inverter loading, and 40% is not out of the question.  Climates where clear and cold conditions occur have to be careful in over sizing the solar array for the inverter past 20%, colder temperatures let the solar array produce more power and can cause production losses if the inverter isn't sized correctly.  It is this designer’s opinion that 120% inverter loading will work just about anywhere, and should be the standard when pairing a solar array with an inverter.

Friday, February 7, 2014

Solar System Design - String Sizing

When designing a solar system, the most important calculation is determining the length of the string of solar panels.  Solar inverters and charge controllers have set voltage windows that have to be met by a string of solar panels whose voltage can vary as much as 40 – 60% throughout the year.  With low string voltages, operation is less efficient and the system can be in danger of shutting off during hot conditions.  Design a string voltage too high and cold sunny conditions could put the inverter into an overvoltage fault mode which shuts the inverter down.  Solar designers have to hit the “sweet spot” where their string voltage will always fall within their equipment’s voltage window while maximizing the string length for more efficient operation.  This is done by designing solar strings based on the upper voltage limit of the inverter or charge controller.

Effect of Temperature on String Voltage

At its basic level, higher temperatures drop voltage and lower temperatures raise voltage in electronics.  For the solar designer, this means string voltage is at its highest when the temperature is coldest, and the extreme low temperature is used to design the solar string.  There are two methods for calculating solar string voltage based on temperature, both outlined in NEC 690.7(A) Maximum Photovoltaic System Voltage:

1)      …Maximum photovoltaic system voltage for that circuit shall be calculated as the sum of the rated open-circuit voltage of the series-connected photovoltaic modules corrected for the lowest expected ambient temperature …. The rated open-circuit voltage shall be multiplied by the correction factor provided in Table 690.7…
2)      When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum photovoltaic system voltage as required by 110.3(B) instead of using Table 690.7.

The first method calls for using NEC Table 690.7.  To use the table, take your solar panel’s open circuit Voltage rating (Voc), found in the data sheet, and multiply it by the temperature correction factor based on your lowest expected ambient temperature.  The lowest expected temperature can be the record low temperature which can usually be found online.  For example, in Albuquerque, NM, our record low temperature is -17o F. Converting to C puts it at -27o C, with a corresponding adjustment factor of 1.21.  This means for Albuquerque I would multiply the solar panel’s Voc by 1.21 to find the maximum design voltage for string sizing.  Assuming a typical 60-cell solar panel with a Voc of 37V, the maximum design voltage is 44.77V.

The second method requires using an equation and referencing the temperature coefficient of voltage found on the solar panel data sheet, but it gives a more exact answer than using NEC Table 690.7.  The temperature coefficient of Voc is usually between -0.3 and -0.4 % per degree C/K, but it varies from panel to panel.  The equation for temperature effect on string voltage is:

Design Voltage = Voc *(1 + TVoc * (Design Temperature - 25o C))

Using a temperature coefficient of -0.33 %/C and the Voc and low temperature used in method 1 (37 Voc, -27 C), the design voltage becomes:

                Voc * (1  + (-0.0033 * (-27 - 25)) = Voc * (1 + 0.1716) = 43.35V

Note that the voltage determined using voltage coefficient is slightly lower than that found using the NEC table.  The NEC table is the more conservative and less exact method to use, but it’s also a little easier than using the temperature coefficient, which gives an exact answer for the extreme minimum temperature and solar panel.  Per NEC 690.7 (A), the temperature coefficient method should always be used if the temperature coefficient of voltage for the solar panel is known, which it usually is from the equipment data sheet.

Record Low vs. Minimum Dry Bulb Temperature

If you continue reading 690.7(A), there is an informational note on what data can be used for the low temperature in string sizing calculations:

Informational Note: One source for statistically valid, lowest-expected, ambient temperature design data for various  locations is the Extreme Annual Mean Minimum Design Dry Bulb Temperature found in the ASHRAE Handbook — Fundamentals.  These temperature data can be used to calculate maximum voltage using the manufacturer’s temperature coefficients relative to the rating temperature of 25°C.

While it’s a mouthful, the gist of it is if you have access to the American Society of Heating, Refrigeration, and Air Conditioning Engineers (AHSRAE) handbook or to their temperature data, you can use the extreme minimum dry bulb for the low calculation instead of the record low temperature.  This temperature is always higher than the record low.  For example, in Albuquerque, NM, the record low temperature is -17o F, while the extreme minimum dry bulb temperature is 10o F.  If I run it through the voltage coefficient equation again, the design voltage becomes 41.52V.  This is the difference between a string of 13 and a string of 14 on a 600V input solar inverter, so the improvement by using this data can be significant.

Sometimes, your exact location isn’t available in the ASHRAE data tables.  In this case, either select the closest site with similar latitude and elevation, or take an average of surrounding sites to approximate the minimum dry bulb temperature at that location.  If you want to know the minimum dry bulb temperature for your location for solar design but don’t have access to an ASHRAE Handbook – Fundamentals, someone may be able to look it up for you….

Effect of Mounting Method on String Voltage

Sometimes the solar system needs to be designed with shorter strings that are close to the lower bound of the equipment voltage window, and you need to confirm that the system will work in the hottest conditions instead of the coldest.  The voltage coefficient equation and NEC Table 690.7 are both only usable for maximum voltage calculations.  This is because maximum voltage calculations are able to make the assumption that the solar equipment’s temperature is equal to the ambient air temperature, as the low design temperature typically occurs in the hour before sunrise.  For the minimum voltage, the solar array needs to be considered when it’s at its hottest, when it’s producing power and the sun is shining on it.  At this point, the equipment can be much, much hotter than ambient temperature due to the direct solar radiation it receives.

How do you figure out the design hot temperature for a minimum voltage calculation?  To start, go back to weather data for your location and find the average high temperature for the hottest month of the year.  This will become the design temperature in a new temperature coefficient calculation of voltage.  Next, based on the solar mounting method, select the temperature rise that will be added directly to this value.  These are common values for temperature rise that the solar industry uses.

Mounting Method
Temperature Rise
< 10o on a flat roof
36o C
> 10o on a flat roof
34o C
Flush mount, pitched roof
32o C
Ground mount
30o C
Pole mount
29o C

Under optimal conditions (pole mounting), the solar array is assumed to be 29o C hotter than ambient temperature, or 82o F hotter.  It only gets worse the closer you install to the roof, as air circulation decreases and the array temperature steadily climbs.
While the max voltage calculations called for using the open circuit voltage, with minimum the “max power” voltage, or Vmp, needs to be used.  Coupled with the temperature rise factor, the minimum voltage equation becomes:

Design Voltage = Vmp *(1 + TVoc * (Design Temperature + Temperature Rise - 25o C))

Using an average high temperature of 95o F (35o C) with a solar panel Vmp of 30V, here’s an example of the minimum voltage of a solar panel installed on a flush mount:

                Vmp * (1  + (-0.0033 * (35 + 32 - 25)) = Vmp * (1 - 0.1386) = 25.84V

This is much, much lower than the 41.5V calculated earlier for the maximum solar panel voltage.  It highlights the importance of temperature effects on your minimum string size.  Based on these numbers, if a solar inverter with a minimum voltage of 200V were considered, a string of 7 would fail under hot operating conditions, while a string of 8 would continue to work.

Friday, January 31, 2014

System Design with 1000V 3-phase string inverters

Much has changed in the solar PV industry in the past five years.  Solar panel efficiency has grown incredibly while prices have dropped so much that incentives are hardly needed to make solar projects financially viable.  New solar mounting technology allows for automatic grounding from the solar panel, with universal clamps and advanced ballasted and penetrating designs.  However, by far the biggest advances have been in inverter technology.  Inverter efficiency has steadily increased, voltage windows have gotten wider, and dual MPPT inputs are now a standard.  By far the biggest advance has been the approval of a true 1000V DC voltage window.  Coupled with dual MPPT inputs and a wide input voltage window, commercial system design is more flexible and adaptable than it has ever been.

1000V Strings

1000V strings are a direct improvement over 600V strings and the most visible improvement in new system design.  Nominal string voltage increases 66% and allows for smaller wire sizes in the array.   #10 wires can be used for string runs over three hundred feet long and still have a voltage drop under 1.5% for 9A strings.  This allows you to get away with one DC wire size for most projects, allowing you to keep things simple for your purchasers and installers.  In the past, inverter placement was more restrictive because long string runs needed to be eliminated to keep wire costs from getting out of control.  Now, inverter location is more flexible thanks in part to higher voltage and efficiency in the string wiring.

Size & Weight

The other factor that makes the new 1000V inverters so adaptable in placement is their size and weight.  Older central three phase inverters were extremely heavy and required a concrete pad for mounting/wiring them from the bottom.  Even a 30kW three phase central inverter could weigh over 1000 pounds and require a structurally engineering concrete pad be installed.  By comparison, a new 30kW 1000V 3 phase string inverter will weigh as little as 120 pounds and can be wall-mounted on a roof by two installers without any additional equipment.  You can either distribute 1000V inverters throughout the array or centralize them at one location, whatever is best for the project and its requirements.  This allows for much more flexibility when it comes to planning your install.

Reduced complexity

A new feature of the 1000V three phase string inverters is integrated string combiners in their wiring boxes.  This can be done because they have such a high input voltage, it reduces the number of strings required to reach rated power, so they’ve fit enough fused inputs into the inverter to handle almost any installation.  Because they are transformer-less, fuses are required on the positive as well as the negative conductors, so the manufacturers are making it easier for everyone by including integrated positive and negative fuse blocks.  Since the string combiners are already included in the inverters, external DC combiner boxes and inverter-integrated recombiners are eliminated from the design. 

Reduced BOS costs

Inverter-integrated string combiners save money two ways.  First, combined DC runs require larger wires that are exponentially more expensive than the wiring used for individual strings.  Using more of the same type and size wire usually allows you to save money by purchasing in bulk while also saving by only purchasing smaller wire.  Second, external DC combiners and inverter-integrated recombiners are both eliminated from the design.  For commercial projects, these are not cheap.  Specialized DC combiner boxes with integrated disconnects can be over $1000 each, and recombiners can cost several thousand dollars depending on the fuses or breakers required.  While specialized DC electrical equipment is eliminated from the design, it forces the inclusion of something to combine the three phase AC outputs of the inverters.  Three phase AC has significantly less line loss than DC, a cost savings on wire.  Additionally, three phase distribution equipment is less expensive, more available, and easier to work with for your electricians.  By moving the power distribution from the DC side to AC, your electricians and installers will thank you for giving them something more familiar and safer to work with.


1000V three phase inverters have unmatched adaptability for system design.  Dual MPPT inputs allows for two different string sizes in the solar array for each inverter.  In multi-inverter installations, which are the design standard with these inverters, you can have several different string sizes because each inverter’s DC input is isolated.  Pair the dual MPPTs with a wide voltage window, and these inverters can handle almost anything.  Some 1000V three phase inverters have DC voltage windows spanning over 800 volts, and can handle string sizes from 12 to 24 solar panels.  With a huge voltage window and two MPPT inputs to play with, the options available for the system designer and these inverters are amazing.


Designing with 1000V three phase inverters isn’t without its limits.  They are restricted to “inaccessible” solar projects, which means roof-mounted, solar carports, or utility projects that are behind the fence and not a part of the consumer grid.  Commercial ground mounts are not included, but you may be able to get an exception.  Some local municipalities or AHJs will not approve 1000V inverters in any solar project, so check what local requirements are before proceeding with a design with 1000V inverters.  The other big knock against these inverters is they are designed for a 277/480V grid only, no 208V models are on the market currently.  This can be overcome by purchasing an external step-down transformer from 480 to 208V.  Since so much money is saved in wire and DC equipment, a transformer can be added to a 1000V design and it will still be cheaper than a competing 600V central design without a transformer.


By eliminating larger wires and external combiner boxes, you are effectively simplifying the entire installation and reducing complexity and opportunity for error.  Replacing combined string runs with string wires makes wire pulls easier and saves money by allowing bulk purchases of smaller wire.  Installing various DC enclosures and managing/organizing wire sizes can add up over the course of a large project, and by using 1000V string inverters labor and equipment costs are directly and noticeably reduced.  Moving electrical distribution to AC instead of DC makes things easier, cheaper, and more effective.  Wide voltage windows and dual MPPT inputs make these inverters a no-brainer whenever you can use them.  It’s truly a solar revolution that we are living in, and 1000V three phase string inverters take full advantage of that.

Friday, January 24, 2014

Performing a Solar Site Analysis

The site analysis is a critical junction in the process of selling and installing a solar system.  Proper information gathering leaves your installation team with everything they need to know to prepare for the install.  Leave out the wrong piece of info and there could be last minute design changes, emergency trips for more equipment, and an overall lack of professionalism in the eyes of your customer.  A good site analysis will give you all of your design information in an efficient and structured manner and leave a lasting impression on your customer.


The roof analysis begins by noting the roof type and condition.  If it’s a flat roof, make sure you get the roofing material correct.  This is important for ballasted systems (common on flat roofs) where roof pads may be needed to protect the roof from sharp metal edges in the solar racking.  If it’s pitched, the same question on roofing material needs to be answered.  Shingle roofs are most common, but if it’s a metal roof you need to note whether it’s corrugated, pro-panel, or a standing seam roof.  Tile roofs can be flat tiles or curved Spanish tiles, each require their own specialized attachment.  Getting the roof right will ensure you get the proper roofing protection, whether that is flashings, roof pads, or tile hooks. 

Obstructions can come from a variety of sources.  Parapet height needs to be measured for southern roof edges.  Vents, pipes, skylights, and other roof obstructions need to have their location noted if they will affect the solar window in your planned mounting area.  Rule of thumb is 2.5 times the height of the obstruction is the length of the shadow it will case on the winter solstice, read my blog post on calculating shade spacing for a more in-depth analysis.  One way to make measuring obstruction locations easier is to use two measuring tapes or one tape and a long string.  Lay the string/tape out in a perpendicular or parallel line from your obstruction then use your measuring tape to take measurements off of this straight line.  This way, complicated areas and weird geometry can be accounted for.

One thing you can do to save time on the roof is do “roof triage” where you focus your measurements and analysis to the areas of the roof you know will fit solar, and ignore/do a cursory examination of the rest of the roof.  This can be especially relevant if there are a large obstructions off-property shading the roof like trees or buildings, or if there are so many roof obstructions an array just won’t fit there.

Sometimes for roof mounted jobs the AHJ/permitting office will require framing diagrams of the underlying roof structure, even for flat roofs.  To ensure you have this information, take pictures of the attic or refer to the building plans to find the framing direction and spacing on the roof.  This also helps you plan out your roof penetrations and framing layout for your solar system.

Something many installers forget when they’re looking at a roof is how they plan to run wire and conduit within the solar array and back to the point of connection.  Sometimes you are forced to break the array up over multiple roof sections, and running strings or branch circuits across the roof can get expensive.  It’s very important to consider the conduit run and location, it can be a very visible and expensive element on the roof unless properly designed. 


An analysis of a site for a ground mount is pretty straightforward.  Begin by looking for sufficient roof for the solar array.  The big thing to look out for is a septic system or leech field which is usually placed in an open area directly adjacent to the house (sound familiar?).   Underground lines and pipes all need to be accounted for in the array location as well, and avoided at all costs.  Make sure the point of connection on the building is known, because the trenched wire run back to that location needs to avoid these as well.  Something that may not be on your radar when doing a ground mount is the water table and drainage.  If there’s a drainage path or high water table, a solar array cannot be installed on the ground there.  Lastly, the utility drop and any easements on the property are off limits for solar, so stay away!

The ground and substrate material needs to be considered if you are making ground penetrations/footings, solid rock or dense clay may require specialized groundwork equipment to install your system.  You should be noting what grading or groundwork may be required in any case, so when the install happens the site is properly prepared for the solar array.

BOS (Balance of System)

BOS is the rest of the solar equipment needed for an installed system to function properly.  For the purpose of this post, BOS is AC inverters, disconnects, and meters.  The first thing to look for on the outside of the building is the location of the Main Distribution Panel and Billing Meter.  After that, start looking for working space for all of your electrical equipment.  Usually, there will be a solar inverter or subpanel, a single throw safety switch, and a meter socket that all need to fit on a single wall.  Things that need to be avoided are gas lines, water canales and spigots, AC units, existing electrical equipment, and doors and windows.  Additionally, the southern and western exposures should be avoided if possible.  These building faces soak up the sun during the hottest part of the day and can adversely affect system performance by reducing voltage and efficiency of the wiring.

Most AHJ’s/utilities will have specific requirements for BOS location and placement, follow their requirements above all else. Many of these requirements have to do with equipment line of sight to the billing meter/point of connection and accessibility of equipment for utility workers.  Couple these requirements with avoiding gas and water lines, and you can find yourself with little to no space to install your BOS on some jobs.  In those cases, you may have to make an external “equipment rack” out of metal strut to mount your solar equipment, an expense that has to be accounted for.


The tools you should bring to a site visit can change based on the type of roof and nature of the solar project.  For most any job, you’ll want a measuring tape or two to quickly dimension areas and locations of equipment.  A ladder and a way to quickly secure it to your vehicle are needed for any roof mounted project (sometimes two ladders are needed for large/tall roofs, make sure you have enough!).  It may not seem like much, but having a screwdriver or two is a big help for opening stubborn electrical panels.  For electronics, a camera is your safest bet for gathering effective site information.  A calculator is good to have, and if you can get one an Inclinometer is perfect for measuring roof pitches and drainage slopes.  Most of these can be found in modern smart phones and internet enabled devices, they are one of your best tools on the roof.  Finally, no discussion on site analysis would be complete without talking about the Solmetric Suneye.  If you can afford one, nothing is better at providing a comprehensive snapshot of nearby shading issues for the solar array.

Customer Interaction

This is the big one, the one that can differentiate you from the competition and make your customer’s solar installation experience one they’ll remember fondly.  The customer has been talking to their salesperson about a solar system, and now there’s a stranger at their house on top of their roof and poking through their electrical boxes.  Many times what was sold to the customer does not match up with what can be installed on their building, and it’s your job to help guide them to the final design and show them how it will work.  They’re paying thousands of dollars for an elaborate electrical system that they may not fully understand, so you have to be prepared for their requirements and suggestions.  Start by reading the customer, seeing how picky they are and how they respond to your installation plan.  Any reasonable suggestions or requirements they make should be followed as long as they are happy with the finished product and it follows local requirements.  If you can’t meet their requests, try to manage their expectations and explain why you have to do it a certain way.  As long as you are courteous and respectful, your point should be well-received.

Friday, January 17, 2014

Wire Sizing Tools and Resources

Hello ladies and gentlemen!  To wrap up my extended series on wire sizing and voltage drop in solar systems, I want to share some wire sizing tools/resources for calculating voltage drop that can help make your life easier.

Online Wire Sizing Configurators

This is an extremely simple voltage drop calculator that assumes no conditions of use similar to my examples for voltage drop in the previous post.  It allows you to input any voltage into it, which makes it unique among the free online voltage drop calculators I checked out.  It’s simple but easy to use and understand.

This calculator lets you select from a long list of preset voltages, and gives you the choice of DC, AC, or AC three phase.  The issue with this calculator is the list of voltages is very arbitrary, there are “holes” in its selection that prevent it from being a comprehensive tool.  Other than that, it lets you adjust conductor temperature to include derating due to conduit fill and ambient temperatures.  It also lets you select your conduit type.  All in all, it’s a very good calculator for AC voltage drop and an OK calculator for DC. 

Online Wire Sizing & Voltage Drop Information Resources

These are some of the resources I have used in the past for learning about voltage drop and wire sizing.  The information in these articles is extremely useful and in-depth.  I owe a great deal of my knowledge on this subject to them.

Thursday, January 16, 2014

Episode 5 pt 3: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material).  This post will cover selecting the correct wire size based on your maximum solar current and calculating the voltage drop based on that.  I will be heavily referencing the NEC 2011 codebook throughout this post.

Conditions of Use

Before getting into selecting wire sizes, I want to cover conditions of use.  These are factors that need to be applied to the maximum current we calculated earlier based on environmental conditions.  The most obvious condition of use is ambient temperature, found in Table 310.15(B)(2)(a).  Based on most sources, the average high temperature for the location should be used.  This can be obtained from the ASHRAE handbook (the 2% value) or determined from weather data available online.  The other conditions of use are more complicated.  Conduit fill is an important consideration for large systems and long conduit runs, it is found in Table 310.15(B)(3)(a).  This is because wiring will generate heat during operation and the hotter it gets inside the conduit the worse the voltage drop will get.  Conduit distance above roof for roof mounted systems is very important as well, and can make the conductors even hotter, reference Table 310.15(B)(3)(c) for more information.  For the purpose of this post, conduit fill and conduit distance factors will both be assumed as 1 for power production.  Ambient design temperature will be fixed at 85° F, which gives a modifier of 1 on our current calculation for simplicity.

Wire Sizing

From my previous submission, maximum DC solar current is based on the solar panel’s Isc rating multiplied by 1.56.  The maximum AC solar current is the inverter’s maximum continuous current multiplied by 1.25.  Taking these maximum calculated currents, proper wire sizes for them can be selected using NEC 2011 wire sizing methods and tables.  These methods will assume conductors and terminals are rated for 75° C.  The first table to reference is 310.15(B)(16), where the 75° C Copper column will be used to select wire size based on the maximum calculated current.  For example, for a solar panel Isc rating of 8.5A, our maximum DC current is 13.26A.  Using table 310.15(B)(16), the minimum wire size #14 AWG.  For a solar inverter with a rated current of 25A, maximum AC current will be 31.25A.  This results in #8 AWG wire.  Conditions of use, if we had them, would be applied here to the overall ampacity of the wire to derate it and check against our maximum calculated current.  Conveniently, all of our conditions of use for this example are fixed at “1”.

Now that the wire size has been found for the maximum calculated current, it needs to be checked against worst case voltage drop in the conductor for the designed length of the circuit.  If the voltage drop would be too high the wire has to be upsized to the next size to correct for it.  The maximum voltage drop is 1.5% for DC and 1.5% for AC circuits based on my previous post. 

Calculating Voltage Drop

The heart of voltage drop calculations is Ohm’s law, which reads V (Voltage) = I (Current) x R (Resistance). The units are Volts, Amps, and Ohms, respectively.  Voltage drop calculations are based on values for Resistance (Ohms) per 1000 feet found in NEC tables in Chapter 9, Table 8 or 9 (DC circuits Table 8,  AC circuits Table 9). The value we need is for stranded, uncoated copper conductors.  This changes the formula from V = I x R to Voltage Change = I x (R / 1000’) x Total Circuit Length
Written another way, it can be expressed in terms of 1-way circuit length and converted into a percentage value:

Equation for Voltage Drop based on NEC 2011 Chaper 9 Tables 8 & 9

Going back to my example in Wire Sizing, let’s consider a DC and an AC wire run for a 100 foot distance and calculate the voltage drop for each.  For DC, we have:

#14 AWG Wire  300V String         I = 13.26A            L = 100 feet         R = 3.26 (NEC Chapter 9 Table 8)

VD = 2 x 100 x 13.26 x 3.14 / 1000 = 8.33V               8.33V / 300V = 2.78% Voltage Drop

Based on our maximum desired DC voltage drop of 1.5%, this obviously will not do.  We need to upsize the wire to get a better voltage drop.  Rerunning the numbers for a #12 wire gives:

VD = 2 x 100 x 13.26 x 1.98 / 1000 = 5.44V               5.44V/ 300V = 1.75% Voltage Drop

Close, but not quite what we wanted.  Obviously a #10 AWG wire is the way to go, but just to be sure:

VD = 2 x 100 x 13.26 x 1.24 / 1000 = 3.42V               3.42V / 300V = 1.10% Voltage Drop

That’s got it right there!  A #10 AWG wire will give us ~1.1% voltage drop for a 300V circuit over 100 feet (In general, I’ve found that a #10 wire is best for solar string wiring for most roofs).  Now that we’ve figured out DC voltage drop, AC voltage drop is pretty simple.  Using Table 9 of Chapter 9, we need to use the values in the column labeled “Alternating-Current Resistance for Uncoated Copper Wires”.  Going back to our earlier example:

#8 AWG Wire     240V AC               I = 31.25A            L = 100 feet         R = 0.78 (NEC Chapter 9 Table 9)

VD = 2 x 100 x 31.25 x .78 / 1000 = 4.875V               4.875V / 240V = 2.03% Voltage Drop

Again, we’re close but not quite where we want to be for this AC wire run.  Upsizing the wire to #6 results in:

VD = 2 x 100 x 31.25 x .49 / 1000 = 3.06V 3.06V / 240V = 1.28% Voltage Drop

Based on our examples, a #10 wire will give us a 1.1% voltage drop for a 300V, 8.5A string of solar panels over 100 feet.  A #6 wire will give us a 1.28% voltage drop for a 240V, 25A solar inverter over 100 feet.

Wire sizing and voltage drop calculations are an extremely important consideration for any solar designer or installer, and it’s essential to understand the fundamentals behind them.  The calculations and examples in this post have been kept simple on purpose, the subject can get complicated in a hurry when multiple conditions of use need to be applied.  In my next post, I will share some of my favorite tools and rules of thumb for calculating wire size and voltage drop.


NEC 2011
Conditions of Use: 310.15(B)(2), 310.15(B)(3)
Wire Ampacity: Table 310.15(B)(16)
Wire Resistance: Chapter 9 Tables 8 & 9

Monday, January 13, 2014

Episode 5 pt 2: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material).  Today I’m going to go over how to calculate your maximum current for a designed voltage drop in a solar system.

Based on last week’s article, the amount of voltage drop to design for in a solar PV system is usually around 3% unless otherwise specified.  This should be split between the DC and the AC conductors, so 1.5% maximum DC and AC voltage drops are the end goals.  If microinverters are being used, there are no DC conductors that need to be sized.  In that case, the AC voltage drop should still be kept below 1.5% to help with grid synchronization.  Now, it’s time to break out the NEC 2011 codebook and start figuring out how to size our wires.

Current Calculations

Before getting into voltage drop and circuit distances, the first thing we need to figure out is how much current to design for.  Looking at solar panel and inverter data sheets, there’s a few different values for current we can use.  On the DC side, our starting point will always be the solar panel’s Isc current rating at STC.  This is based on NEC article 690.8, Circuit Sizing and Current.  We are concerned with 690.8(A)(1) and 690.8(B)(1)(a) for DC current calculations, which state:

690.8(A)(1): The maximum current shall be the sum of parallel module rated short-circuit currents multiplied by 125 percent.

690.8(B)(1)(a): Overcurrent devices, where required, shall be rated to carry not less than 125 percent of the maximum current calculated in 690.8(A).

While these talk about overcurrent devices, our circuits will have to be sized based on these same requirements.  Now, looking at the solar panel’s Isc rating, it has to be multiplied by 125% per 690.8(A)(1).  Then to size wire for it, an additional 125% multiplier needs to be added per 690.8(B)(1)(a).  Taken together, the DC current multiplier for wire sizing is 1.56 times Isc.

On the AC side, the inverter’s rated continuous current (make sure it’s for the correct AC voltage!) needs to be used for wire sizing calculations.  Like on the DC side, there’s a factor that needs to be applied to find our AC wire sizing current.  Looking at the same NEC article, the inverter output circuit current can be found using 690.8(B)(1)(a) and again referencing 690.8(B)(1)(a), which together state:

690.8(A)(3): The maximum current shall be the inverter continuous output current rating.

690.8(B)(1)(a): Overcurrent devices, where required, shall be rated to carry not less than 125 percent of the maximum current calculated in 690.8(A).

Unlike on the DC side, only one 125% multiplier needs to be applied on the AC current to find our maximum current for wire sizing.  The final AC current multiplier for wire sizing is 1.25 times rated continuous current.  Next, we’ll select wire sizes and calculate voltage drop based on our solar current calculations.  To keep post length down, I’m breaking it up into multiple submissions.

Saturday, January 4, 2014

Episode 5 pt 1: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material). Today I’m going to look at designing for voltage drop and how to design for it so your solar array functions properly.

The maximum voltage drop you should design for in a system is not set in stone.  For commercial systems, it can be specified in the bid documentation, but for smaller systems you are usually left trying to figure out the best value that will deliver the most energy without breaking the bank on wire.  While the National Electric Code (NEC 2011) does not explicitly provide a maximum permissible voltage drop for solar PV systems, you can find a reference point in Articles 210 and 215 in fine print notes.  They state that overall voltage drop should not exceed 3% for branch or feeder circuits and 5% overall.  Based on this, most solar systems are designed to never exceed 3% overall voltage drop as a feeder circuit, with no more than 1.5% voltage drop on the AC side.

The reasons you want to keep voltage drop low in your solar system are multiple. 

First, the obvious reason is wasted energy and wasted dollars. A solar array is a major investment, and if an upgrade from #12 to #10 wire will provide a significant decrease in voltage drop it will pay for itself throughout the life of the system.  A difference between 1% and 2% voltage drop between your inverter and the interconnection is a difference of 50W for a 5000W system, and it quickly adds up the larger it gets.

On the DC side of the inverter, you need to hit a defined voltage window.  If you size your solar strings to the lower end of that range, a high voltage drop could put you outside of the maximum power point tracking window of the inverter. This is at its worst in hot weather, when voltages are already low and the inverter is hot.  This is also some of the best production time for solar PV systems all year, optimum performance is critical and your DC wiring needs to be sized with that in mind.

AC voltage drop is possibly more important than DC for solar PV systems.  The issue is one with the electric grid and your inverter.  Your service voltage can vary depending on several conditions including weather and time, and the inverter is constantly working to match up with that voltage.  This is why on inverter data sheets you see an AC voltage range the inverter is capable of producing, typically -12% / +10%.  For a 240V inverter, this gives an upper voltage limit of 264V.  Grid voltages as high as 250V or higher have been seen for 240V services. iIf your AC wire was sized with a 5% voltage drop your inverter would see 262.5V, almost its limit and at risk of going into fault.  This is why low AC voltage drop is extremely important, and most commercial bids and lease agreements require 1% or less.

Now that you have a guideline for how much voltage drop you want to design for in your solar system, how do you do it?  Next week I’ll be digging into the NEC 2011 codebook for wire sizing methods and solar current calculations.  I'll also share some of my favorite online/mobile tools and rules of thumb to help make this as smooth and painless as possible.  Happy New Year!