Solar panel calculator
How many panels it takes to cover your electricity use, how much roof they need, and how long they take to pay for themselves.
Panels needed
0 panels
How it works
system size (kW) = yearly use (kWh) x share / output per kW panels = system size x 1000 / panel watts roof area = panels x 21 sq ft (a 400 to 430 W panel is about 5.6 x 3.7 ft) payback (years) = cost after rebates / yearly savings
Output per kW is the number that changes most from house to house. It depends on where you live, which way the roof faces, its pitch and how much shade falls on it. Savings assume every kWh the panels make is worth your full electricity price. That holds with one-for-one net metering, but not under rules like California's NEM 3.0, where power sent back to the grid earns much less.
Typical yearly output per kW
| Region | kWh per kW per year |
|---|---|
| Desert Southwest (Phoenix, Las Vegas) | 1,600 to 1,800 |
| California, Colorado, Utah | 1,450 to 1,600 |
| Texas, Florida, the Southeast | 1,300 to 1,450 |
| Midwest and Northeast | 1,100 to 1,300 |
| Pacific Northwest | 1,000 to 1,150 |
These are rough figures for an unshaded roof facing south at a moderate pitch. A roof facing east or west makes about 15% less, and shade from trees or a neighbouring roof can cost far more than that.
Getting the real number for your roof
The RoofTools report works it out per roof plane from the address. It takes a typical weather year for the location (NREL's NSRDB), traces the shade that nearby trees and buildings cast across each plane hour by hour using the LiDAR surface, and prices the output at the local utility's average rate from the EIA. It then lays out panels plane by plane with fire-code setbacks from the edges and ridge.
Tax credits
The 30% federal residential clean energy credit does not apply to systems installed after 2025, so this calculator starts with no credit. Many states and utilities still offer rebates or performance payments; put their total in the rebates field.
More roof calculators
Skip the tape measure
Type an address and RoofTools measures the roof from public LiDAR: every plane's pitch, direction and sloped area, the total in squares, and sun and shade for a solar layout. Then you can fix any plane in the editor.
Want someone to do the job?
Tell us where and when, and we'll put you in touch with a roofer or solar installer near you. No account needed.