Utility Watchdog in San Diego

Rate your Photovoltaic System in Order to Calculate your Solar Rebate

Ok this section may make you eyes glaze over a little, but to be able to compare proposals from different installers and to understand how the CSI rebate is calculated, you need to know a little about how PV systems are rated. Take a deep breath, keep calm, and read slowly. There may be a quiz afterward.

The first rating is called DC-STC which stands for Direct Current - Standard Test Conditions. This is also referred to as the nameplate rating. This measures how many watts of direct current are generated under lab test conditions. Solar panels usually give a DC-STC rating in their model number. It will be something like XYZ180 and would have a DC-STC rating of 180 watts. Multiply the panel rating times the number of panels and you get your system DC-STC Rating. This is important because DC-STC is used by the PVWatts Calculator to predict how much electricity your system will produce in a given day, month, or year.

The next rating you need to know is DC-PTC which stand for DC-PVUSA Test Conditions. This is a test by an independent lab and predicts performance in conditions closer to real life. The DC-PTC rating is always lower than the DC-STC rating. This is important because this is used to calculate the CEC -AC rating.

The CEC-AC rating starts with the DC-PTC rating, corrects this for azimuth and tilt, and multiplies this by the inverter efficiency. This gives a reasonable way to compare a PV systems capability to produce the alternating current we use in our homes. This is important because your CSI rebate is calculated based on the CEC-AC rating of your system.

Ok, this brings us to one last rating, the AC rating. In addition to inverter efficiency, there are other losses in converting the potential direct current produced by the solar panels to the alternating current we use in our homes. This includes things like wiring losses, dust on the solar panels, and manufacturing tolerances. Estimates are made of all the losses in the system to determine a DC-to-AC Derate Factor. Multiplying the DC-STC rating by the derate factor gives you the AC rating. This is important because this is what is actually used to predict how much the usable electricity PV system will produce.

Ok, you made it through. You did so well, we won’t have a quiz.

About UCAN

UCAN has represented the interests of San Diego County utility customers since 1983. UCAN focuses its efforts on the rates and services of San Diego Gas and Electric Company, telecommunications utilities and the City of San Diego Water Department.