Building & decorating / Solar
Solar Panel Calculator: How Many Panels Do I Need?
Estimate how many solar panels you need to cover your electricity use from your monthly consumption, the peak sun hours where you live, the panel rating, and system losses, with the system size, roof area, and expected yearly production. A simplified estimate.
Solar Panel Calculator: How Many Panels Do I Need?: Each panel produces its rating × peak sun hours × (1 − losses) a day. Using 900 kWh a month is 29.6 kWh a day; a 400 W panel with 4 peak sun hours and 14% losses makes 1.38 kWh a day, so 22 panels are needed, an 8.8 kW system on about 42 m² of roof, producing about 11,000 kWh a year. Runs 100% locally in your browser with zero server file uploads.
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A simplified estimate: each panel makes its rating × peak sun hours × (1 − losses) a day. Peak sun hours are the equivalent hours of full 1,000 W/m² sunshine averaged over the year, roughly 2.5 to 3.5 in northern Europe and 4 to 6 in sunny regions; national solar maps and PVGIS give values for your address. Losses of about 14% cover wiring, the inverter, heat, and dirt. Shade, roof direction, and tilt change the result, so get a site survey before buying.
Direction and tilt
In the northern hemisphere, panels facing south at a tilt close to the latitude produce the most over a year; east and west roofs give about 15 to 20% less but spread production across the day.
Savings
To value the energy at your tariff, enter the yearly kWh in the electricity cost calculator.
How to use it
- Enter your monthly electricity use from a bill.
- Enter the peak sun hours for your area and the panel rating.
- Read the number of panels, the system size, and the roof area.
Privacy & limitations
Everything is calculated in your browser.
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Frequently asked questions
What are peak sun hours?
The average number of hours a day of sunshine at full strength, 1,000 W/m², that would give the same energy; they are lower than daylight hours and vary with place and season.
Where do I find mine?
From national solar maps or the European Commission's PVGIS tool, which works out the expected production for an exact address, roof direction, and tilt.
Will this cover my use all year?
On average, but winter production can be a third of summer's at high latitudes; without batteries or net metering you will still draw from the grid at night and in winter.
Free tool · runs in your browser · no account required