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

For example, a 205 watt Sharp ND-205U1 module is rated at 180.6 watts. This revised wattage is referred to as PTC watts, or Performance Test Conditions watts.

Because the CEC rebate is predicated upon final AC watts, and because these AC watts ultimately determine how much useable electricity comes out of the system, it’s important to understand them. So far, we’ve learned that PTC watts are the performance, or real-world, rating of the module and that STC watts are in a perfect world.

All of the power generated from the solar modules is converted from Direct Current to Alternating Current (DC to AC). This conversion is performed by the inverter which has its own efficiency rating. Like solar modules, the CEC has efficiency rating they use:

http://www.consumerenergycenter.org/cgi-bin/eligible_inverters.cgi

Let’s use an example of 18 Shell 175 watt modules and a PV Powered 2800-XV inverter to do some calculations.

The 175 watt modules are rated at 158.3 watts (PTC). 18 modules multiplied by 158.3 equals 2849 watts. The PV Powered 2800-XV inverter has an efficiency rating of 94.5% thus the AC size of this system would be 2692 (2,849 multiplied by .945). After all that, the CEC determines the rebate amount on the current rebate level multiplied by the AC watts.

This chart may help explain…

Example: 18 Shell 175 watt PC175 and PV Powered 2800-XV inverter

Module wattage = 175

CEC rating = 158.3

# of modules = 18

18 x 158.3 =2849

Inverter efficiency = 94.5%

2849 x .945 = 2693

2693 x $2.60 per watt rebate =$7,001

That’s all there is to it. Once you know STC, PTC, and CEC AC, solar becomes much less mysterious and you’ll have a lot more confidence when shopping for a PV system.