Solar Panel Calculator: How Many Panels Do I Need?

Work out how many solar panels and what system size would cover your electricity use. Enter your yearly usage from your bills, your local sun hours, the panel rating and a loss allowance.

How to use this tool

1. Add up twelve months of kilowatt-hours from your bills, or multiply a typical month by twelve.
2. Enter your average peak sun hours. PVWatts or your installer can give a site-specific figure.
3. Enter the panel wattage you are considering and a loss allowance for heat, wiring, inverters and shade.
4. Choose the share of your use to cover.

Worked example

A home using 10,500 kWh a year with 4.5 peak sun hours and 18 percent losses needs about 7.8 kW of panels. With 400-watt panels that rounds up to 20 panels, an 8 kW system, producing an estimated 10,775 kWh a year.

Understanding your results

The panel count is rounded up, so estimated production slightly exceeds your target. Roof space, orientation, shading and your utility’s sizing rules often set the real limit. If the count will not fit, higher-wattage panels or a partial offset are common answers.

Common mistakes to avoid

• Using one summer month instead of a full year.
• Ignoring shade from trees or nearby buildings.
• Oversizing beyond what your utility allows or credits.
• Forgetting planned changes such as an EV or heat pump that will raise usage.

Next steps

Check the economics with the Solar Payback Calculator.

Frequently asked questions

How many solar panels does it take to power a house?

Divide your yearly kilowatt-hours by the production of one kilowatt of panels in your area, then by the panel wattage. Many homes need roughly 15 to 30 modern panels, but your own bills are the only reliable starting point.

What are peak sun hours?

Peak sun hours express a day’s sunlight as the number of hours at full-strength sun. Four to six is common across much of the United States, with lower values in winter and in cloudier regions.

Do bigger panels mean fewer panels?

Yes. Higher-wattage panels produce more per panel, so fewer fit the same target, which helps on small roofs.

This tool is for planning and education only. It does not store your entries. Work on home wiring must be done by a licensed electrician. Read our disclaimer.