Solar Panel Count for Energy Needs
Panels = daily kWh need / (panel-W × sun-hours × derate / 1000).
Result
Panels needed
20
8.0 kW system, ~360 ft² roof.
- Daily energy need30 kWh
- Panel rating400 W
- Sun hours/day5
- Derate factor0.77
- Energy per panel1.540 kWh/day
- Panels20
- System size8.00 kW
- Roof area needed360 ft²
Step-by-step
- Energy/panel/day = 400 × 5 × 0.77 / 1000 = 1.540 kWh.
- Panels = 30 / 1.540 = 19.48 → round up = 20.
How to use this calculator
- Daily kWh from utility bill / 30.
- Pick panel rating (modern: 350-450W).
- Find local sun hours (NREL).
About this calculator
Solar sizing rule: panels needed = daily kWh / (panel-watts × sun-hours × 0.77). 30 kWh/day need (typical US home) × 400W panels × 5 sun-hrs × 0.77 derate ≈ 20 panels = 8 kW system. Sun hours vary geography (NREL maps): Phoenix 6.5, Boston 4.0, Seattle 3.6. NREL system derate (0.77) accounts for heat, dust, inverter losses, wiring.
Frequently asked
US average: 30 kWh/day = ~10,800 kWh/year. Varies hugely: small apt 10/day; large house 60-80/day.
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