This solar panel calculator sizes a home solar system: from your annual electricity use and local sun hours to the number of panels, system size in kW, and the roof area it will occupy.
Assumes a 15% system-loss factor (inverter, wiring, soiling, temperature) folded into production, and ~17.5 sq ft per panel. Shading, roof orientation and tilt move real output ±20% — installers model those with your actual roof.
The Sizing Formula
Each panel produces wattage × peak sun hours × 365 × 0.85 (the 0.85 covers real-world losses) per year. Divide your target production by that figure and round up — panels are integers. “Peak sun hours” is not daylight length; it is the day’s sunlight compressed into equivalent full-sun hours (a good day in Arizona ≈ 6.5; a gray Pacific Northwest average ≈ 3.5). What the generated electricity is worth lives in our Electricity Cost Calculator. Part of our Ecology Calculators collection.
Worked Example (Verified)
10,000 kWh/year home, 4.5 peak sun hours, 400 W panels, 100% offset:
- Per panel per year: 400 × 4.5 × 365 × 0.85 = 558 kWh
- Panels: 10,000 ÷ 558 = 17.9 → 18 panels = a 7.2 kW system
- Roof: 18 × 17.5 ≈ 315 sq ft of clear, preferably south-facing roof — a rectangle about 15 × 21 ft
- At an 80% offset target instead: 15 panels (6.0 kW) — partial offsets are common where net-metering rules reward self-consumption over export
Before You Trust Any Quote
- Your bill’s kWh is the ground truth — pull 12 months, not the “average home.” An EV in the driveway adds ~2,500–3,500 kWh/yr (plan for it if one is coming).
- South-facing at latitude-ish tilt is ideal in the northern hemisphere; east/west roofs produce ~15–20% less but spread generation across the day.
- Shade is nonlinear: one chimney shadow can drag a whole string down (module-level electronics — optimizers/microinverters — exist precisely for this).
- Panels degrade ~0.5%/year; year-25 output ≈ 88% of year-1. Sizing slightly large today is a legitimate strategy.
Related Tools
- Carbon Footprint Calculator — the emissions these panels displace
- Electricity Cost Calculator — valuing the output
- Ecology Calculators — the full collection
Frequently Asked Questions
How many solar panels do I need for a house?
For an average US home using ~10,500 kWh/year in a decent sun region: roughly 18–20 modern 400 W panels (7–8 kW). The calculator personalizes it with your usage and sun hours — the two inputs that move the answer most.
What are peak sun hours?
The equivalent hours of full-strength (1,000 W/m²) sun your location averages per day: ~3.5 in Seattle, ~4.5 in Chicago/Denver, ~6.5 in Phoenix. It is an energy measure, not sunrise-to-sunset duration.
How much roof space does solar need?
About 17.5 sq ft per panel plus access margins — the example 18-panel system needs ~315 sq ft of usable roof plane. Ground mounts dodge the roof question entirely where land allows.
Should I offset 100% of my usage?
Depends on net-metering: where exports are credited at full retail rate, 100% (or slightly less) is standard; where exports pay wholesale, smaller systems sized to daytime consumption often pay back faster. Your utility’s tariff decides.
How long do solar panels last?
Performance warranties typically guarantee ~85–90% output at 25 years; inverters are the shorter-lived component (10–15 years for string inverters). Panels frequently outlive their warranties by a decade.
Is this solar panel calculator free?
Yes — free, instant, no sign-up. For a binding quote, an installer will model your roof’s shading and tilt precisely.