How to use the solar panel calculator
- Enter your electricity use. Use your average monthly bill, or better, the average kWh from the last 12 bills. Summer and winter use can differ a lot.
- Check the electricity rate. Divide your bill total by the kWh used to get your real rate, including delivery charges.
- Pick your sun hours. Peak sun hours are the equivalent hours of full sunlight per day. They range from about 3.5 in cloudy northern states to 6 or more in the desert Southwest.
- Choose how much of your bill to offset. 100% aims to cover all your use. Go lower if roof space is limited or your utility pays little for exported power.
Solar sizing formula
System size (kW) = yearly use ÷ (sun hours × 365 × system efficiency)
Panels = system size × 1,000 ÷ panel wattage, rounded up
Payback (years) = system cost ÷ yearly savings
System efficiency (often called the derate factor) covers losses from inverters, wiring, heat, dust and shading. 75% to 85% is typical.
Worked example
A $150 monthly bill at $0.17 per kWh means about 882 kWh a month, or 10,588 kWh a year. With 4.5 peak sun hours and 80% efficiency, the home needs 10,588 ÷ (4.5 × 365 × 0.8) = 8.06 kW. That is 21 panels of 400 W (8.4 kW), about 440 sq ft of roof. At $2.80 per watt the system costs about $23,500 and saves about $1,800 a year, a payback of roughly 13 years.
How many solar panels does a house need?
| Monthly use | 4.0 sun hours | 5.0 sun hours | 6.0 sun hours |
|---|---|---|---|
| 600 kWh | 16 panels | 13 panels | 11 panels |
| 900 kWh | 24 panels | 19 panels | 16 panels |
| 1,200 kWh | 31 panels | 25 panels | 21 panels |
| 1,500 kWh | 39 panels | 31 panels | 26 panels |
Based on 400 W panels and 80% system efficiency.
What affects solar savings
- Net metering rules. If your utility credits exported power at the full retail rate, savings are highest. Many states now pay less for exports, which makes batteries or a smaller system more attractive.
- Roof direction and shade. South-facing roofs produce the most in the US. East and west roofs produce roughly 10% to 20% less.
- Incentives. Under the 2025 federal tax law, the 30% residential clean energy credit no longer applies to systems paid for after December 31, 2025. Check current federal rules plus state and utility rebates, and enter them as a percentage above.
- Electricity price trends. Rates that rise 2% to 4% a year make solar pay back faster than this calculator's flat-rate estimate.
Want to know which appliances drive your bill? Use the electricity cost calculator. Replacing the roof first? Size it with the roofing calculator.
Estimates for planning only. Get quotes from several licensed installers for a site-specific design.
Frequently asked questions
How many solar panels do I need to power a house?
An average US home using about 900 kWh a month needs roughly 16 to 24 panels of 400 W, depending on how many peak sun hours your area gets.
How much does a solar panel system cost?
Installed residential solar often costs around $2.50 to $3.50 per watt before incentives. An 8 kW system at $2.80 per watt is about $22,400.
How long does it take for solar panels to pay for themselves?
Typically 7 to 15 years in the US. Payback is faster with high electricity rates, strong sun, full net metering and local incentives.
How much roof space do solar panels need?
A modern 400 W panel is about 21 square feet. A 20-panel system needs roughly 420 square feet of usable, unshaded roof, plus setbacks required by fire codes.
What are peak sun hours?
The number of hours per day when sunlight averages 1,000 watts per square meter. Five hours of strong midday sun can equal a full day of weaker light. US averages range from about 3.5 to 6.
More home project calculators
Last updated October 2026.