The PV panels in my system are made by the REC Group, the model is REC215AE-US. Each PV panel is rated at 215 watts under STC conditions, and 187 watts under PTC conditions.
STC stands for Standard Test Conditions. Under this standard, the panel output is given for a solar radiation level of 1000 watts/sqmeter, and for a panel surface temperature of 25C (77F). This is a pretty optimistic rating, and in actual service, panels will not produce the STC rated power except under very very favorable conditions.
PTC stands for PVUSA Test Conditions. They test the panels with an ambient air temperature of 20C ( 68F) with a 1 meter/sec wind speed at 10 meters (33 ft) above the ground, and with a solar radiation level of 1000 watts/sqmeter. The PTC ratings are typically about 12% lower than the STC ratings, and are closer to real world performance. Most of the difference has to do with the fact that surface temperatures of PV panels under PTC conditions will typically be about 50F higher than the ambient temperature, or 68F + 50F = 118F -- this is quite a bit higher than the STC assumed surface temperature of 77F, and the panel performance drops at the temperature rises.
But, panel performance varies a lot depending on your typical temperatures, whether the panels get some air movement around them for better cooling, clearness of the air, elevation, ... While my panels have a PTC rating of 187 watts, they will routinely produce 200 watts on clear winter days -- and, they would go somewhat higher if the inverter did not limit their output. But, we are at 5000 ft elevation with those clear Montana skys and cold temperatures :)
This page gives STC and PTC ratings for hundreds of panels that have been approved for use in California.
This paper shows the actual measured output for a number of actual PV installations in California...
Other factors that effect PV panel output:
- Temperature -- a 1 degree C rise in temperature of the surface of the PV panel causes about an x% drop in power output. This is why allowing some ventilation room around and under the panels can improve output.
- Soiling -- dirt on the panels reduces output -- keep them clean!
- Age -- output goes down of the order of x% per year.
- Shading -- PV panels are very sensitive to even partial shading -- the shadow from something as small as a plumbing vent stack can make a significant difference. This is why site surveys are so important.
In one case, I found that by brushing away a narrow band of snow along the bottom of the panels, that output immediately increased by a factor of 4!
Gary November 18, 2009