Driving home today it struck me as clear as anything in my life how lucky I was. Lucky because I get to work in an industry like solar, that is as all American as it comes. Solar appeals to the very roots of being an American. Independence, freedom, and the ability to control your own destiny.
Lucky because I spend most of my work week on the phone answering questions about home solar kits helping homeowners and off-grid independent folks make the solar leap. When I am not working in technical services on the phone, I cannot wait to put my hands on a home solar emergency generator other grid tied or off-grid solar kit heading out the door.
You see part of my job also involves verifying a customer’s solar kit works as advertised, and that the order goes smoothly. Each day I cannot wait to put my hands on the off-grid solar kit and connect the solar panels to the emergency solar generator or other kit and watch it work. Many folks look at solar and understandably see it as a mystery. Once you have a chance to walk folks through some of the basics, it becomes much clearer.
Solar panels that come from established manufactures have a name plate rating on the back of the panels that references STC wattage. STC stands for “Standard Test Conditions”. This is a phrase that is thrown around a lot by home solar salespeople and solar sites. I suppose there needs to be some standard measurement of comparing the performance of one solar panel to another, but the STC comparison is almost misleading.
Misleading because the panel STC rating has little to do with real world performance. Case in point, I set up a emergency solar kit heading out the door yesterday to verify it worked. It was around 11:00 AM and the panels were set at about a 45-degree angle. I did not measure the true Azimuth, but the solar panels were facing about 180 degrees. I then measured the output of the two 180-watt Trina solar panels. The two solar panels were producing about 210 watts of electricity. You might first think the panels were defective, no they actually were working just about as expected.
Here is the scoop: STC Rating Vs PTC; STC in an acronym for “Standard Test Conditions”. All solar panels are rated in Watts. The watt rating is how much power (amps times volts) the panel will produce in full sunlight at 25 degrees C (77F). This is the industry standard (STC) for all solar panel ratings. Solar panel manufactures have long used this test standard which is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and a 25 degrees C. cell temperature.
PTC is an acronym for “PV-USA”. The PV-USA test conditions was developed at the PV USA test site at the University of Davis, California for standards established by the California Energy Commission that are considered closer to real world conditions (Real World Vs STC factory test conditions). The PTC rating test is 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second. In California, solar PV panels must be tested and rated independently at the PV USA test facility at the University of Davis (CA) to be considered for rebates.
The ambient temperature rating (PTC) is generally considered a better real-world standard than factory conditions because silicon solar cells average about 20 degrees C. above ambient temperature in the real world, cell voltage drops as temperature increases. A module’s power output in real life conditions is lower than the power measured at the panel manufacturing factory where cell temperature is maintained at a controlled 77 degrees F. (25 C).
STC Vs PTC Cell voltage drops about 0.08 volts per degree C. in environments which exceed 25 degrees C. That means an STC rating of 17 volts can actually become a PTC (PV-USA) rating of 15 or 16 volts. Using Ohm’s Law, volts times amps is equal to watts which equals power, so a reduced voltage, means reduced watts.
CEC is an acronym for the California Energy Commission. About 10 years ago the California Energy Commission drove the development of closer to real world standards that would be more appropriate when considering rebate programs. It is common these days to see both STC and PTC power ratings measured in watts on most commonly accepted solar panels. The California Energy Commission also maintains a list of approved solar panels on its website for public viewing. In order for a rebate to qualify in California, applications for rebates must include the PTC standard rating not STC since PTC represents closer to real world conditions.
Neither PTC nor STC account for all “real-world” losses. Actual solar products will produce lower outputs due to soiling, shading, module mismatch, wire losses, inverter and transformer losses, shortfalls in actual nameplate ratings, panel degradation over time, and high-temperature losses for arrays mounted close to or integrated within a roofline. These loss factors can vary by season, geographic location, mounting technique, azimuth, and array tilt.
My concern is, when we sell a home emergency solar, grid tied, or off-grid kit, does the homeowner really expect a 180 watt STC rated solar panel to produce 180 watts? The answer is, not if I have done my job right.