Solar Panel Calculator (How Many Panels Do I Need?)
Panels needed to offset annual electricity use, from panel wattage, peak sun hours and system losses.
Formula
Annual kWh per panel = W/1000 × peak sun hours × 365 × (1 − losses). Panels = ⌈annual use ÷ kWh per panel⌉.
Worked example
Inputs: Annual usage (from your bills) = 12000 kWh; Panel rating = 400 W; Peak sun hours/day (look up in PVWatts) = 5 h; System losses = 14 %.
| Kwh per panel year | 627.8 |
|---|---|
| Panels | 20 |
| System kw dc | 8 |
FAQ
Why 14% losses?
14% is the default system-loss assumption in NREL PVWatts; change it if you have a design estimate.
Is this a quote?
No. Roof orientation, shading, inverter choice and utility rules change real output; get a site-specific design.
Sources
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Last reviewed 2026-10-11.