How many solar panels do I need?

Updated October 2026

The number of panels depends on three things: how much electricity you use, how much sun your location gets, and how much power is lost between the panels and your appliances. This is the same method the system size calculator uses.

The five steps

1. Daily use. Take the monthly kWh from your bill and divide by 30.4. Example: 900 kWh ÷ 30.4 = 29.6 kWh a day.
2. Peak sun hours. These are not daylight hours. They count the hours of full-strength sun, and most places get between 3 and 6. Use 4.5 if you are unsure.
3. System losses. Heat, wiring, the inverter and dust usually cost 15 to 25 percent. Use 20 percent.
4. Array size in kW. Daily use ÷ (sun hours × (1 − losses)). Example: 29.6 ÷ (4.5 × 0.8) = 8.2 kW.
5. Panel count. Array watts ÷ panel watts, rounded up.

Example: a home that needs 8.2 kW

Larger panels mean fewer panels, but a 1000 W panel is a large-format module that often needs a ground mount or two people to lift.

Adjustments that matter

• Off-grid: add about 15 percent, because energy passes through the battery and you cannot sell or borrow power from the grid.
• Roof space: plan on roughly 4.75 m² (51 ft²) per kW. The 8.2 kW example needs about 39 m² of unshaded roof.
• Direction and shade: panels should face the equator and be free of shade between mid-morning and mid-afternoon.
• Future use: if you plan an electric car or air conditioning, size for that load now.

Next step: enter your own numbers in the calculator, then compare panel types on the solar panels page.