The yearly number is a model
Kilowatt-hours measure energy. The yearly total in a proposal is a forecast, not a measurement of what your future system will produce. The estimate depends on tilt, compass direction, location, equipment, and shading. Change any input and the lifetime savings story moves with it.
Ask how shade was accounted for
Trees, chimneys, neighboring roofs, and winter sun angles can reduce energy production. Ask how the proposal accounts for them. If an independent model estimates less energy, the projected savings may be lower and recovering the purchase cost may take longer.
What to compare
Look at the yearly estimate, the assumed shading loss, tilt, and direction. If two quotes use different roofs in the model — one with shade, one without — they are not quoting the same system even if the panel count matches.
Compare estimates using the same inputs
PVWatts estimates energy output using inputs including system capacity, tilt, azimuth, and losses. Its output is a model result, not a production guarantee. PVWatts calculator and model inputs (U.S. national laboratory calculator; accessed September 18, 2026)
Illustrative example: one 8 kW proposal estimates 12,000 kWh per year and another estimates 10,800 kWh. That is a 1,200 kWh difference, or 10% of the first estimate. Ask what inputs explain it before assuming either is right.
- Confirm the same DC system size and roof planes are modeled.
- Compare shade, losses, tilt, direction, and weather assumptions.
- Ask whether annual energy refers to system output or energy delivered after other losses.
- Keep the forecast separate from any written production guarantee.