Solar Panel Calculator — Size, Cost & Payback
Estimate a home solar system from your electricity use and local sunshine. See the system size, number of panels, roof area, cost after incentives, yearly savings and how long it takes to pay for itself.
System size
How to size a solar system
- Find your monthly kWh on your electric bill; average the last 12 months if you can.
- Enter peak sun hours for your area: roughly 3.5–4 in the Northeast and Northwest, 4–5 in much of the Midwest and South, and 5.5–6.5 in the Southwest.
- Enter a quoted price per watt, any rebates, and your electricity rate.
How the solar estimate works
System size in kilowatts = yearly kWh ÷ (peak sun hours × 365 × 0.8). The 0.8 factor accounts for inverter losses, wiring, heat and shading. A home using 900 kWh a month in an area with 4.5 sun hours needs about an 8.2 kW system, or roughly 20 panels of 420 W. Each modern panel takes about 20 square feet of roof.
Payback comes from the electricity you no longer buy. Savings grow as utility rates rise and shrink slightly as panels degrade (about 0.5% a year). Net metering rules matter a lot: if your utility credits exported power at less than retail, savings are lower unless you use the power yourself or add a battery.
Important for 2026: the 30% federal Residential Clean Energy Credit ended for homeowner-owned systems placed in service after December 31, 2025. Enter any state, local or utility incentives you qualify for. Leases and power purchase agreements work differently. Get several quotes; installers will model your roof, shading and orientation precisely.
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Frequently asked questions
How many solar panels do I need?
Divide the system size by the panel wattage. An 8 kW system needs about 19–20 panels of 400–420 W.
Is there a federal solar tax credit in 2026?
Not for homeowner-owned systems: the 25D credit ended for systems placed in service after December 31, 2025. State and utility incentives may still be available.
What are peak sun hours?
The equivalent number of hours per day of full-strength sunlight (1,000 watts per square meter) at your location.
How long do solar panels last?
Most are warrantied for 25 years and keep producing beyond that at slightly reduced output.