How solar panel sizing actually works
Sizing a solar array is a straightforward energy-balance problem: you need to generate, on average, as much electricity as you consume. The calculator above converts your monthly electricity bill (in kWh) into a recommended system size (in kW), a panel count, and an expected annual output, using the same core method solar installers use for a quick pre-quote estimate.
The formula
First, convert monthly usage to a daily average: daily kWh need = monthly kWh ÷ 30. Then divide by how much usable sunlight your location gets and how efficiently your system converts that sunlight into delivered power:
System size (kW) = daily kWh need ÷ (peak sun hours × derate factor)
Once you know the system size, two more numbers follow directly:
- Panel count = system size in watts ÷ wattage per panel (rounded up, since you can't buy a fraction of a panel).
- Annual production (kWh) = system size (kW) × peak sun hours × 365 × derate factor.
Peak sun hours
A "peak sun hour" is a period during which solar irradiance averages 1,000 watts per square meter — the standard test-condition intensity used to rate panels. It is not the same as hours of daylight; a long overcast day may have 10 hours of daylight but only 2 peak sun hours. In the United States, daily averages range from about 3–3.5 in the Pacific Northwest and Northeast, to 4–5 across most of the country, up to 6–6.5 in the Desert Southwest (e.g., Phoenix, AZ). You can look up your specific location using NREL's PVWatts tool or your utility's solar resource maps.
The derate (system efficiency) factor
A solar system rarely delivers 100% of its nameplate (DC) rating as usable (AC) electricity. Losses come from inverter conversion (~2–4%), wiring resistance, panel soiling and shading, temperature effects on panel output, and slight power-rating tolerances. Combined, these typically total 14–25% of losses, so a "derate factor" of roughly 0.75–0.86 (75–86%) is standard for a rough estimate — this calculator defaults to 85%, a commonly cited planning value for a well-installed, unshaded system.
Worked example
A home using 900 kWh/month sits at 30 kWh/day. With 4.5 peak sun hours and an 85% derate factor: 30 ÷ (4.5 × 0.85) ≈ 7.84 kW. With 400 W panels, that's 7,840 ÷ 400 ≈ 19.6, rounded up to 20 panels. Expected annual output: 7.84 × 4.5 × 365 × 0.85 ≈ 10,950 kWh/year — enough to offset roughly a full year of that home's usage (900 × 12 = 10,800 kWh).
Roof area
Standard residential panels measure roughly 3.25 ft × 5.5 ft (about 17.8–18 sq ft each), so the calculator multiplies panel count by ~18 sq ft as a rough space estimate. Actual roof area needed is usually higher once you account for setbacks, obstructions (vents, chimneys), and orientation limits.