Is my solar system underperforming?

1. Your sunlight. Your zip’s first three digits pick one of 102 reference locations. For each we use monthly solar data from NREL’s National Solar Radiation Database (2005 to 2015), retrieved through the European Commission’s PVGIS 5.2 tool.
2. A reference system. PVGIS models a 1 kW array tilted 20° facing south, including panel heat, with 14% system losses. That 14% is the default loss budget in NREL’s PVWatts: soiling, wiring, mismatch, inverter, a little shade and downtime.
3. Your roof. If you tell us pitch and direction, we adjust for how much sunlight that plane gets each month compared with the reference, using a standard sky model. If not, the range covers common roofs from east- to west-facing.
4. A range, not a number. Expected = size × reference output × days × roof adjustment. The range adds that month’s year-to-year weather spread in the NSRDB years and 10% for everything this simple model leaves out.

It cannot see heavy shade, snow, a cloudier-than-usual month or panels that degraded with age. NREL notes that a single month can differ from a typical-year estimate by as much as 40% from weather alone, so use this as a first check, not a verdict. Sources: NREL NSRDB, EC JRC PVGIS, PVWatts on interpreting results. Not covered: Alaska, Puerto Rico and military zips.