How solar sizing works
Installers size a system from one number: your annual electricity usage in kWh. Everything else is geography and physics.
The sizing equation
kW needed = annual kWh ÷ (peak sun hours × 365 × derate)
- Annual kWh — from your utility bills, all 12 months. A home using 900 kWh/month needs 10,800 kWh/year.
- Peak sun hours — your location's average daily solar intensity, 3.0 (Alaska) to 6.5 (Arizona). State averages are a starting point; your roof's tilt and azimuth move the real number.
- Derate (~0.80) — the real-world haircut for inverter losses, wiring, dust, heat, and panel mismatch. A 7 kW array behaves like a 5.6 kW one.
Why panels come in whole numbers
kW ÷ panel wattage rarely lands on an integer, so installers round up to whole panels. That slight oversize is normal and harmless under net metering — the extra kWh become bill credits.
What the equation ignores
Future usage changes (EVs, heat pumps, kids leaving), panel degradation (~0.5%/year), and rate structure changes. If you're buying an EV within a few years, size for it now — adding panels later costs more per watt.
This is a planning method, not a design. Try the system sizer, then verify with NREL's free PVWatts calculator and 2–3 installer quotes.