Grid-Tied PV System Performance Factors: Page 2 of 4

Intermediate

Inside this Article

Cleaning solar modules
Grid-Tied PV System Performance Factors
A remote meter
A remote meter can help you keep track of how well your array is performing.
Utility meter
Taking readings from the PV system production meter can help you identify system problems, but only if you have previous records to compare to.
A temperature gun
A temperature gun checks infrared radiation to give quick readings of cell temperature.
Measuring cell temperature
Placed against the back of a module, a sensor (attached to a meter) can be used to measure cell temperature. Be sure to take readings on several spots and average the results.
Pyranometer sensor
To accurately determine the irradiance striking your PV array, be sure the pyranometer sensor is on the same plane as the modules.
PV module label
While this label does not show the PV module’s power tolerance, it does give the minimum figure, which computes to be 91% of the rated power (154.7 ÷ 170.0).
Dirty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Dusty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Partial shading of a PV module
Shading can negatively affect your PV system’s output. For testing purposes, be sure your array is in full sun.
Misshapen junction box
Warped, misshapen boxes or insulation indicate electromechanical problems with one or more PV modules.
Burn marks
Burn marks indicate electromechanical problems with one or more PV modules.
Monitoring display
Finding individual underperforming PV modules is possible with module-level tracking technologies, such as microinverters or DC optimizers.
Measuring the array’s operating voltage
Measuring the array’s operating voltage at the positive and negative terminals, with the array connected to the inverter.
A clamp-on meter
A clamp-on meter is used to measure the current while the array is connected to the inverter.
Cleaning solar modules
A remote meter
Utility meter
A temperature gun
Measuring cell temperature
Pyranometer sensor
PV module label
Dirty PV module
Dusty PV module
Partial shading of a PV module
Misshapen junction box
Burn marks
Monitoring display
Measuring the array’s operating voltage
A clamp-on meter

Module temperature. The higher its cell temperature, the lower the PV module’s voltage will be. With lower voltage comes less output (power). Even if it’s 25°C (77°F) outside, in direct sun modules will heat up much higher than the ambient temperature. Systems utilizing ground- and pole-mounted systems, and ones on elevated racks on low-slope roofs benefit from air circulation; in full sun, they tend to be 20°C to 25°C above the ambient air temperature. Systems that are flush-mounted on pitched rooftops have much less air flowing under the modules and can be 35°C to 40°C above ambient air temperature.

We could measure the air temperature with a thermometer and estimate the module cell temperature from the temperature adders above, but this would not be very accurate. A much better way is to use an infrared thermometer on the back of the module, if possible, since the glass front can cause skewed readings (see “Tools of the Trade” sidebar). It’s a good idea to take the temperature in a couple of places throughout the array and average the results. If the array is easily accessible, temperature sensors will provide the most accurate readings. Just be sure the sensors are measuring the module cell temperature, and not the ambient air or module frame temperature.

In our example, even though the air temperature is a moderate 18°C (65°F), we measure the module cell temperature at 53°C (127°F). Next, we need to convert this value into a temperature factor to use in our final calculation. First, find the difference between the module cell temperature and STC (25°C), converting Fahrenheit to Celsius:

Temperature difference = 53°C - 25°C = 28°C

Next, calculate the derate factor for the power we expect to lose from the increased temperature (it is preferable to use the specific module’s temperature coefficient of power instead of the estimated -0.5% per °C, which is quite conservative):

Temperature derate factor = 1 + (-0.5% per °C difference × 28°C) = 0.86

Our module cell temperature is higher than at STC, and the temperature factor should be less than one since the overall array power will be lower than at STC. If the module cell temperature is less than 25°C, then we should expect a temperature factor greater than one.

Irradiance. The more sunlight (irradiance) hitting the modules, the more current they will produce. Irradiance is measured with a handheld pyranometer.  It needs to be placed at the exact same tilt angle and orientation as the PV array, ideally lined up adjacent to a module.

Many things affect the irradiance levels at the PV array, including time of day, season, elevation, cloud cover, and sometimes even reflection from snow. Unlike module cell temperature, irradiance levels can change very rapidly. A cloud passing by the sun can cause levels to spike, drop, and then return to normal (edge-of-cloud effect) in less than a minute! It’s important to look for a window of time, on a clear sunny day, when you can take a couple of readings that you expect to be similar, and record the inverter’s output power between the irradiance readings. Even better, have a friend assist, with one person recording irradiance and another recording inverter output simultaneously.

For our example, an irradiance reading of 875 W per square meter is recorded at 3 p.m. Although the sky is clear, there is a slight haze in the air. This was taken at the same time as the inverter output reading of 925 W (noted above). Since 875 W per square meter is less than the 1,000 W per square meter rating at STC, we should be seeing less power than the STC ratings, and an irradiance factor of less than one. To calculate, we simply divide our reading by 1,000:

Irradiance Factor = 875 ÷ 1,000 = 0.875

While irradiance levels of less than 1,000 W per square meter are common in most of the United States, at higher elevations, and with snow on the ground, higher irradiance levels are possible, yielding an irradiance factor greater than one.

Comments (3)

Justine Sanchez's picture

Hi Gary,
Thanks for posting! Yes a production meter is an extremely valuable asset to any grid-tied PV system. Separate production meters (and faceplate inverter output meters) can give us a good idea if the PV system is meeting energy (kWh) production expectations overtime. But if your kWhs aren't stacking up to what is expected, this article provides the next step in troubleshooting, and describes a method of spot checking the wattage of the system, so that we can determine if there is actually a problem with the system itself, rather than something other such as a cloudier than usual spring or perhaps partial morning or afternoon array shading due to tree growth over time, etc..And yes the method described here does require some specialty tools (ex/irradiance meter and temp gun) which will be common to the installer, but not necessarily to the average homeowner.
Cheers,
Justine Sanchez
Home Power Magazine

gary beckwith's picture

this is an interesting article with some good information for the techies and tinkerers. but for practical purposes, the easiest way to check on your system is to install a separate meter for production if there isn't one already. personally I think it's a good idea to put a production meter on your system even if it isn't required, just for this purpose. then go to the PVWatts website and put in your location, tilt angle, temperature etc, and see what you should be getting every month and per year. the PVWatts calculator already factors in all the derate issues mentioned in this article. So if you're consistently below what PVWatts says, then you should call your installer and have them troubleshoot your system.

Robert Sczech_2's picture

A 3KW system will generate roughly 3 MWH of electricity per year - a market value of roughly $300. If one is really to perform all the maintenance tasks and checks on the system as described in the article and if one values ones own life time at only $10 per hour, then the value of ones human lifetime invested into these activities could easily eat up a significant part of the electricity created.

A personal experience: The state of NJ has mandated the installation of so called revenue grade meters for all solar installation in order to accurately measure the amount of electricity generated. In my case, the process of installing that meter started last July and is still not complete. During the past year many emails and phone calls needed to be made to get the contractor, the permits and the final inspections. The inspection process alone takes many months because of the paperwork requirements and repeated reinspections. If all the human labor in getting these meters installed is all added up, then the gain from renewable does not appear to be that significant. Lots of that gain is lost in the inefficiencies of the full installation process.

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