How it works
Solar panels are sized to the electricity you use in a year. We take your monthly use (or work it out from your bill and price per kWh), then find the system size that produces that much in your sunshine, allowing 20% for real-world losses: heat, wiring, inverter conversion, dust and some shading. Dividing by the panel wattage gives the number of panels, rounded up.
"Peak sun hours" is the sunshine your roof gets, expressed as hours of full-strength sun (1,000 W per m²). It is lower than daylight hours: about 4 to 6 in most of the US, under 3 in the UK. For your exact roof, NREL's PVWatts (US) or the EU's PVGIS (UK, Europe, Australia) give a precise figure.
The formula
yearly use (kWh) = monthly use × 12 system size (kW) = yearly use ÷ (sun hours × 365 × 0.8) panels = system size × 1,000 ÷ panel watts (round up) roof area (m²) = panels × panel watts ÷ 210 (21%-efficient panels) cost = system size × 1,000 × cost per watt
Worked example
A home using 865 kWh a month (close to the US average), 4.5 peak sun hours, 400 W panels.
| Step | Working | Result |
|---|---|---|
| Yearly use | 865 × 12 | 10,380 kWh |
| System size | 10,380 ÷ (4.5 × 365 × 0.8) | 7.90 kW |
| Panels | 7,900 ÷ 400, rounded up | 20 panels |
| Actual system | 20 × 400 W | 8.0 kW |
| Roof area | 20 × 400 ÷ 210 | 38 m² (410 sq ft) |
| Cost before incentives | 8,000 W × $2.60 | $20,800 |
Frequently asked questions
How many solar panels do I need for 1,000 kWh a month?
About 23 panels of 400 W (a 9.2 kW system) with 4.5 peak sun hours a day. In sunnier places such as Arizona you need fewer; in the Northeast or the UK you need more.
How many solar panels does it take to power a house?
It depends on how much electricity you use, not on floor area. A typical US home using about 865 kWh a month needs around 20 panels of 400 W in average sun. A UK home using 2,700 kWh a year typically needs a 3 to 4 kW system (8 to 10 panels).
How much roof space do solar panels need?
A modern 400 W panel is roughly 1.9 m² (about 20 sq ft). Allow extra space for fire-code setbacks from ridges and edges, vents and chimneys, so a usable roof face needs to be noticeably larger than the panel area.
Should solar cover 100% of my electricity?
Not always. Where your utility pays little for exported power, a slightly smaller system that you use mostly during the day can pay back faster. Use the solar payback calculator to compare sizes.
What are peak sun hours?
The day's total sunshine expressed as hours of full-strength sun (1,000 W per m²). Six hours of daylight with weak morning and evening sun might equal only four peak sun hours.
Sources
- NREL PVWatts Calculator for location-specific production (US).
- European Commission PVGIS for the UK, Europe and Australia.
- Installed cost defaults: National average quote before incentives, EnergySage 2026; About £1,500–1,800 per kW installed (MCS data, 2026) (UK); About $0.95 per watt installed after the STC rebate, 2026 (Australia). Checked October 2026.
Related guides
Estimates are for planning only and are not financial advice. Get written quotes before you buy.