Understanding STC, PTC, and CEC AC watts
This guide uses California Energy Commission standards for photovoltaic systems. The rebate calculation, performance ratings, and calculations are all California specific. Those of you who are in other parts of the country may have other incentives and standards. Now let’s unravel the watt standards.
It seems that there's a bit of confusion about how to properly name the size of a system in watts. There is talk of standardizing sizing in the industry to end the confusion. But for now, STC,PTC,CEC,and DC watts can be confusing. The differences are substantial in terms of what you pay per watt, comparing the prices of different vendors, and what the rebate amounts are. Also, it’s important to compare “apples to apples” when shopping for a PV system. Since California is # 3 in the world for photovoltaics, they’ve developed a widely respected standard that we’ll explain here.
Detangling the watt question
There are a few terms that need to be understood about solar electric systems:
STC DC watts-The nameplate rating of a solar module.
PTC DC watts-the rating of a module in real-world conditions as determined by the California Energy Commission.
CEC AC watts-the total PTC DC of solar modules factoring in inverter efficiency. This the number that the rebate is based on.
When a solar modules nameplate says 175 watts, this means that in perfect conditions the module will produce 175 watts of power. In controlled conditions a manufacturer will use something akin to a camera's flash to measure how much power comes out of the module. This measurement is called Standard Test Conditions or STC for short thus the nameplate rating is known as STC watts.
In order to determine truly what a solar module is capabale of in the real world, it’s necessary to test it. The California Energy Commission (CEC) conducts studies on the many different modules using criteria to account for dust, wiring losses, module mismatch, weather, etc. They publish their findings on their website:
http://www.consumerenergycenter.org/cgi-bin/eligible_pvmodules.cgi |
