Solar Panel Count for Energy Needs

Panels = daily kWh need / (panel-W × sun-hours × derate / 1000).

Inputs

AZ/NV: 6-7 hrs. PA/NY: 4 hrs. UK: 2-3 hrs.

NREL standard: 0.77 (heat losses, dust, inverter, wiring).

Result

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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

How much energy does a typical home use?+
US average: 30 kWh/day = ~10,800 kWh/year. Varies hugely: small apt 10/day; large house 60-80/day.
Why "derate" 0.77?+
NREL PVWatts standard. Accounts for module heating (-10%), wiring loss (-2%), inverter (-3%), dust (-2%), aging (-3%).
Battery storage?+
For grid-tied: not needed (net metering). For off-grid: 1-2 days of usage in batteries. Powerwall (13.5 kWh): ~$10k.
Roof orientation?+
Best: south-facing in N hemisphere, 30-45° tilt. Penalty: east/west ~15% less; flat 5-10% less.
Cost?+
~$3/W installed (residential 2025). 8 kW system ≈ $24k before tax credit. Federal ITC 30% (through 2032).

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