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.