Solar Panel Calculator
Estimate the solar system size, number of panels, installed cost, yearly savings and payback period for your home based on your electricity use and local sunshine.
Formulas shown on this page · Last reviewed October 2026
How solar system size is calculated
system kW = (monthly kWh × 12 × offset %) ÷ (peak sun hours × 365 × 0.8)
panels = system watts ÷ panel wattage
The 0.8 factor accounts for real-world losses from inverters, wiring, heat, shading and dust. Your installer will refine this using your roof orientation and tilt.
Peak sun hours by region (approximate)
| Region | Peak sun hours |
|---|---|
| Southwest (AZ, NV, NM, southern CA) | 5.5–6.5 |
| Texas, Florida, Southeast | 4.5–5.5 |
| Midwest, Mid-Atlantic | 4–4.5 |
| Northeast, Great Lakes | 3.5–4.2 |
| Pacific Northwest | 3–4 |
What affects payback
Payback is fastest where electricity is expensive and sunshine is plentiful. Net metering rules matter too: if your utility credits exported power at a low rate, sizing the system closer to daytime use – or adding a battery – can improve returns.
Solar panels by monthly usage
500 kWh/month · 600 kWh/month · 700 kWh/month · 800 kWh/month · 900 kWh/month · 1,000 kWh/month · 1,200 kWh/month · 1,500 kWh/month · 1,800 kWh/month · 2,000 kWh/month · 2,500 kWh/month · 3,000 kWh/month
Frequently asked questions
How many solar panels do I need for my house?
An average US home using about 900 kWh per month needs roughly an 8 kW system in a location with 4.5 peak sun hours – about 21 panels of 400 W.
How much does a solar system cost?
Residential solar typically costs $2.50–$3.50 per watt installed in the US, so an 8 kW system runs about $20,000–$28,000 before any rebates.
What are peak sun hours?
Peak sun hours are the equivalent number of hours per day when sunlight delivers 1,000 W per square meter. They range from about 3.5 in the Northeast and Pacific Northwest to 6 or more in Arizona and Nevada.
Is there still a federal solar tax credit?
The 30% residential clean energy credit (Section 25D) ended for systems installed after December 31, 2025. Some states and utilities still offer rebates – enter them in the incentives field.