AN OFF-GRID PERSPECTIVE
STC, PTC, CEC, CEC-AC What Does It All Mean?
If you’ve spent any time looking at solar panels or doing research on solar power, you’ve come up against many new acronyms. Some of the most confusing aspects of solar power are understanding how much power a solar panel, or more correctly the solar module, will produce. This is especially important when comparing systems and evaluating how much power a system will ultimately produce.
Standard Test Conditions (STC):
So let’s start with the manufacture. Below is a typical spec sheet for a 170,180 and 190-watt solarmodule. When the manufacturer sells this module, they list the maximum power that the module can produce. It’s listed in this table as maximum power or Pmax in watts. If you note under the title (Electrical Performance) they specify that this module be tested under STC or standard test conditions.This is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and a 25 degrees C. cell temperature.
That’s all fine and dandy, but most modules don’t operate in a laboratory. The important wordin the STC rating to look at is cell temperature. It is tested at 25 degrees C. That’s 77 degreesF. Most module cells will operate around 20 degrees hotter than the ambient outside air temperature. That means that this test assumes that the outside temperature is 57 degrees F. So why do they want to test in such a cool environment? Simple, if you look down on the chart you’ll see that the temperature coefficient shows that for every degree above 25 C you’ll lose .45% off of the stated max power.
So let’s do the math: at a roof temperature of 100 degrees, not that uncommon in most of the USA,your 190 watt module is really only producing around 161 watts. That’s a 28% reduction, not so good for marketing.
PVUSA Test Conditions (PTC):
The solar industry needed a real world test of the modules so the PVUSA testing center in Davis, California, developed the PTC rating, which is typically 10-15% lower than the STC rating. The PTC rating represents a more real life condition based upon 1,000 Watt/m2 solar irradiance, 20 degree Celsius ambient temperature, and 1 meter/second wind speed. Note that this test is at 20 degrees C ambient which works out to a cell temp around 40 C or 104 degrees F. The facility in Davis takes modules from different manufacturers and puts them head to head and records the output data, an independent test of output. The PTC rating is great for consumers to look at when shopping for solar panels.
The CEC or CEC-AC watt rating:
The California Energy Commission (CEC) adopted the PTC rating to measure nominal output power of photovoltaic cells or modules to determine the system’s rating in order to calculate the appropriate incentive level. So you may see a module’s power output listed as a CEC rating which is really just the PTC method. The CSI (California Solar Initiative) program uses the CEC method as well. After all these ratings, is this the real measure of power coming from your system? No.
This is where CEC-AC comes into play. We still have to take into account the location of the array geographically, the tilt angle, azimuth of the array and the loss of efficiency going through the inverter. What comes out the other end of all the equipment and is fed into the AC grid power at the home is the CEC-AC wattage rating. The CEC uses this to determine the incentive or rebate a consumer can receive.
So in review, you start with the STC rating as a marketing number that the manufacturers use. Use this number as a general size of the module. The PTC rating will give you a more realistic amount of wattage that a given module will produce. Use this number when figuring out how much DC wattage an array will produce. And finally, because the rebates are predicated on the KW power going into the grid, the CEC-AC wattage is used for figuring out the rebate amount for grid tie customers. The CEC-AC KW can be used as a direct comparison against your utility bill’s daily KW usage. The expected annual CEC-AC KW production can be compared against your actual annual use.