How Many Solar Panels Do I Need?
Short answer: a typical home needs 15–25 panels — but the honest answer comes from three numbers you can find in five minutes: your monthly kWh, your local sun hours, and the wattage of a modern panel. Here's the math, then the shortcuts.
The three-step formula
- Daily usage: monthly kWh ÷ 30. A home using 900 kWh/month needs 30 kWh/day.
- System size: daily kWh ÷ peak sun hours ÷ 0.85 (system losses). With 5 sun hours: 30 ÷ 5 ÷ 0.85 ≈ 7.1 kW.
- Panel count: system size ÷ panel wattage. With 430 W panels: 7,100 ÷ 430 ≈ 17 panels.
"Peak sun hours" isn't daylight hours — it's the equivalent hours of full-strength sun your roof gets per day, averaged over the year. Rough guide: 3–3.5 in northern Europe and the US Pacific Northwest, 4–5 across most of the US and central Europe, 5.5–6.5 in the US Southwest, Australia, and most of India and Africa.
Worked examples (5 sun hours, 430 W panels)
| Monthly usage | Typical home | System size | Panels | Roof area* |
|---|---|---|---|---|
| 500 kWh | Small apartment / efficient home | 3.9 kW | 9 | ≈ 200 sq ft |
| 750 kWh | 2–3 bed home | 5.9 kW | 14 | ≈ 300 sq ft |
| 900 kWh | Average US home | 7.1 kW | 17 | ≈ 370 sq ft |
| 1,200 kWh | Large home / mild EV use | 9.4 kW | 22 | ≈ 480 sq ft |
| 1,600 kWh | Large home + EV + AC | 12.5 kW | 30 | ≈ 650 sq ft |
*Modern residential panels are roughly 21.5 sq ft (2 m²) each. Panel wattages now run 400–450 W; if a quote uses 370 W panels, expect ~15% more of them.
What moves the number most
- Your actual usage — grab 12 months of bills, not one. Summer AC or winter heating can double a month.
- Sun hours — the same house needs ~40% more panels in Seattle than in Phoenix.
- Roof direction and shading — south-facing (northern hemisphere) is the baseline; east/west roofs produce ~15% less; regular shade can cost 20%+.
- Future loads — an EV adds 250–350 kWh/month; a heat pump 300–800 kWh/month in winter. Size for the home you'll have, not the one you had.
Do the math with your numbers
Our calculators run this exact method with your inputs:
- Solar Panel Size Calculator — system size, panel count, and roof area from your usage.
- Solar Output Estimator — what a given system produces where you live.
- Solar Savings & Payback — whether those panels actually pay off.
- Solar Battery Size — add storage for outages or night use.
Then get 2–3 real quotes and compare their assumed production against the estimator — if a quote promises much more than the math supports, ask why.