Solar panel calculator
Estimate how many solar panels you need, the system size in kWp and the roof area from your electric bill usage.
How many solar panels do I need?
Estimate for planning. The final design must be done by an installer, who takes into account tilt, shading and the utility.
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Solar panel calculator: what it is for
Enter your average monthly electricity usage, your area's peak sun hours and the panel wattage. The calculator subtracts the utility's minimum charge and estimates the system size, the number of panels, monthly and yearly generation and the roof area taken up.
It is an estimate for planning before asking for quotes.
How to use
- Type your average usage in kWh.
- Adjust sun hours and panel wattage.
- See the panels, kWp and roof area.
When to use it
- Ask for solar quotes knowing the size.
- Compare installer proposals.
- Estimate savings on the electric bill.
Frequently asked questions
How do I calculate the number of solar panels?
Divide your monthly usage (in kWh) by 30 days, by your area's peak sun hours and by the system efficiency (about 80%). That gives the size in kWp. Then divide by the power of each panel.
What are peak sun hours?
It is the day's solar energy converted into hours of strong sun. Most places get between 3.5 and 6 hours, more in sunny, dry regions and less in cloudy or northern ones.
Why subtract the minimum charge?
Even if you generate all your energy, grid-connected homes usually pay a fixed minimum or connection charge to the utility. In Brazil it is the equivalent of 30, 50 or 100 kWh depending on the connection. It isn't worth sizing the system for that part; choose Don't subtract if your utility has no such charge.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.