Solar panel calculator
Enter the units you use in a month, or a bill and a price per unit. The page shows the system size, the number of panels, a roof area and yearly generation.
- Panels and kilowatts
- Free, no sign-up
Runs in your browser. Nothing you type or open is uploaded. Free to use, no account. More everyday tools
How to use it
- Enter monthly units, or a bill and the price per unit.
- Pick a peak-sun region or type your own hours, plus the performance ratio and panel watts.
- Read the kilowatts, the panel count, the roof area and the yearly total.
From monthly units to panels
Daily use is the monthly total divided by 30. System kilowatts are that daily figure divided by peak sun hours and the performance ratio. The panel count rounds up so the array meets the load. The default ratio is 0.75, which leaves room for inverter loss, heat, dust and wiring.
Peak sun hours are a planning average for the region, from about 2.5 in the United Kingdom to about 5.5 in southern India. Roof area uses about 1.95 m² for a 400 W panel and scales with the wattage. A site survey measures shade and roof direction before anyone orders panels.
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Everything runs in this page and stays on your device. The units you enter stay on this device.
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Questions
How many panels for 300 units a month?
At 5 peak sun hours, a 0.75 performance ratio and 400 W panels, the array is about 2.67 kW, which rounds up to 7 panels.
What are peak sun hours?
The equivalent hours per day when the sun is strong enough to produce the panel’s rated output. A region preset is an annual average you can replace with a local figure.
What is the performance ratio?
The share of the ideal output you still get after heat, dust, wiring and the inverter. 0.75 is a practical planning figure. A very clean, cool site can be higher.
How much roof do the panels need?
About 1.95 m² for each 400 W panel, scaled if you change the wattage. Seven 400 W panels are about 13.7 m².
Can I start from my electricity bill?
Yes. Choose the monthly bill, enter the amount and the price per unit. Units are the bill divided by that price.