How Accurate Are Solar Production Estimates?

Every solar proposal includes some estimate of how much electricity the system will generate. The number is useful, but it is still an estimate based on assumptions about sunlight, shading, equipment, weather, and system losses.

What goes into the estimate

Production models may account for roof orientation, tilt, shading, weather patterns, system losses, inverter efficiency, and panel specifications.

Site-specific modeling is better

A proposal based on a detailed roof and shading assessment is generally more informative than one based only on the home's address and a generic production factor.

Weather creates uncertainty

No model can predict the exact weather your roof will experience over the next 20 years. Production in one year can therefore be meaningfully different from another.

Ask about losses

The headline panel wattage is not the same as the amount of electricity that reaches your home. Temperature, wiring, inverter conversion, shading, dirt, and other factors reduce total output.

Compare competing proposals

The best way to compare estimates is to ask each installer for the same basic information: system size, estimated annual production, assumptions, shading analysis, degradation assumptions, and equipment details.

Beware unrealistic precision

A forecast that claims to know exactly how many kilowatt-hours you will produce decades into the future is giving a false impression of certainty.

Good solar modeling provides a reasonable expected range and explains the assumptions behind it. That's far more useful than a single impressive number with no context.