Solar Panel Calculator
Size an off-grid solar array: enter your daily energy use, peak sun hours, and panel wattage to get the number of panels and array size in kW.
How this calculator works
An off-grid solar array has to cover your daily energy use even after real-world losses. Panels never deliver everything they collect, and the sun is only strong for part of the day.
This calculator takes your daily energy use, pads it by 20% for system losses, then divides by your local peak sun hours to find the array size in kilowatts. It then divides that by a single panel’s wattage and rounds up to a whole number of panels. Enter your daily kWh, peak sun hours, and panel rating to get both figures.
Formula: Array size (kW) = daily kWh × 1.2 (system losses) ÷ peak sun hours; Panels = array watts ÷ panel rating, rounded up.
Worked example
A home using 30 kWh a day, with 5 peak sun hours and 400 W panels:
- Daily energy use: 30 kWh
- Array size = 30 × 1.2 ÷ 5 = 7.2 kW
- Array watts: 7200 W
- Panels = 7200 ÷ 400 = 18 panels
Notes
Peak sun hours vary by location and season, so the same array produces more in a sunny region than a cloudy one, and more in summer than winter. Use a figure that matches your own area rather than a broad national average.
The 1.2 factor covers inverter and wiring losses, along with the smaller drains from heat and dust. It keeps the array sized to actually meet your daily use instead of falling just short.
How to use
Enter your average daily energy use in kilowatt-hours, the peak sun hours for your location, and the wattage of the panels you plan to buy. The calculator returns the array size in kilowatts and the number of panels needed, with losses and rounding already handled for you.
Sizing here is a planning estimate. Confirm your system with a solar installer, and account for local sun hours, shading, and battery needs.
Frequently asked questions
How do I calculate how many solar panels I need?
Start with your daily energy use in kilowatt-hours and multiply by 1.2 to cover system losses. Divide that by your local peak sun hours to get the array size in kilowatts. Then divide the array watts by the wattage of one panel and round up. In the worked example, 30 kWh a day works out to a 7.2 kW array, or 18 panels rated at 400 W each.
What are peak sun hours?
Peak sun hours measure how many hours of full-strength sunlight a location receives in a day, once weaker morning and evening light is added up into equivalent full-sun hours. A place with strong sun might see 5 or 6, while a cloudier region sees fewer. Because it varies by location and season, use a figure for your own area rather than a general average.
Why multiply daily energy use by 1.2?
No solar system delivers every watt its panels collect. Energy is lost in the inverter, in the wiring, from heat, and from dust on the panels. Multiplying your daily use by 1.2 adds roughly 20% to cover those inverter and wiring losses, so the array is sized to actually meet your needs rather than falling short on a normal day.
Why round the number of panels up?
Dividing the array watts by a single panel's rating rarely lands on a whole number. You cannot install a fraction of a panel, and rounding down would leave the array slightly undersized. Rounding up to the next whole panel guarantees the array meets or slightly exceeds your target size, giving a little extra margin for cloudy stretches.
Estimates only. Verify quantities with your supplier before purchasing.