• Sunbelt Solar Energy
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  • Sunbelt Solar Energy

Solar Power, Panels for your Home - All Solar Is Not the Same | SUNBELT SOLAR ENERGY

All Solar Is Not the Same

It seems that there is some confusion about how to properly name the size of a system in watts. There is some talk of standardizing sizing in the solar industry to try to end the confusion. But for now, STC, PTC, CEC, and DC watts can be somewhat confusing. The differences can be substantial in terms of what you pay per watt. And, it is very important to compare “Apples to Apples” when shopping for a solar electric PV system. The California Energy Commission (CEC) has developed a widely respected standard that we will explain here.

View the video below to see the SunPower difference.

These are a few terms that need to be understood about solar electric systems:

STC DC watt • The nameplate rating of a solar module.
PTC DC watt • 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 in a solar electric system,
Factoring in inverter efficiency.

When a solar module’s nameplate states 200 watts, this means that in perfect conditions the module will produce 200 watts of power. In controlled indoor conditions manufacturers will use something similar 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 STC rating is known as STC watts.

In order to determine truly what a solar module is capable of in the real world, it is necessary to test it. The California Energy Commission (CEC) conducts studies on many different solar modules using criteria to account for dust, wiring losses, module mismatch, weather, etc. They publish their findings on their website.

For example, a 225 watt SunPower SPR-225-BLK module is rated at 207.1 watts. This revised wattage is referred to as the PTC watts, or Performance Test Conditions watts.

Because the CEC AC watts ultimately determine how much useable electricity comes out of the system, it is important to understand them. So far we have 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 (DC) to alternating current (AC). This conversion is performed by the inverter which has it’s own efficiency rating. Like solar modules, the CEC has assigned a efficiency rating to each brand and model of inverters. This rating is a percentage of the inverter’s nameplate rating (example 92% or 96%). Please see the “Important Links” page on this site for a link to the Go Solar California site that shows the inverter test ratings and the solar module test ratings.

Let ’s use and example of 22 SunPower 225 watt modules and a SMA 5000 inverter to do some calculations.

The 225 watt modules are rated at 207.1 watts (PTC). 22 modules multiplied by 207.1 equals 4556.2 watts. The SMA 5000 inverter has an efficiency rating of 95.5%, thus the AC size of this system would be 4351.17 (4556.2 multiplied by .955). After all that, the CEC wattage for this system would be 4351.17 watts. This same system has a 4950 watt STC nameplate rating.

That is all there is to it. Once you know STC, PTC, and CEC AC, solar becomes much less confusing and you will have more confidence when shopping for a solar electric PV system.