Solar Panel Calculator
How many solar panels do you need? Enter your monthly electricity use, the sun hours at your location, and the power of the panels you are considering to get a rough size of the system and what it could produce.
A rough estimate
Solar panels needed
19
Check the highlighted fields to see your result.
- System size
- 7.60 kW
- Yearly production
- 11,096 kWh
- Share of your use
- 103%
- Yearly savings
- 1,728
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
The calculator turns your monthly use into a daily figure, then finds the power the system must have to produce that much on an average day, given the peak sun hours and the system losses. Dividing by the power of one panel and rounding up gives the number of panels. It then recalculates the yearly production with that whole number of panels, so you can see how much of your use it covers.
The formulas
System kW = (monthly kWh × 12 ÷ 365) ÷ (sun hours × (1 − losses))
Panels = system kW × 1,000 ÷ panel watts, rounded up
Worked example
A home uses 900 kWh a month, gets 5 peak sun hours, uses 400 W panels, and expects 20% losses.
- Daily use: 900 × 12 ÷ 365 = 29.6 kWh.
- System size: 29.6 ÷ (5 × 0.80) = 7.40 kW.
- Panels: 7,400 ÷ 400 = 18.5, rounded up to 19 panels (7.6 kW).
- Yearly production: 7.6 × 5 × 0.80 × 365 = about 11,100 kWh, which is 103% of the 10,800 kWh used.
Sources and cautions
This is a simple estimate. For a more detailed one that uses real weather data for your address, try the freePVWatts calculatorfrom the National Laboratory of the Rockies (NLR, formerly NREL). Whatever you use, ask a solar installer to check your roof, the shade, and the local rules before you decide.
Frequently asked questions
How many solar panels do I need for my house?
It depends on how much electricity you use, how much sun your location gets, and the power of the panels. A home that uses 900 kWh a month, with 5 peak sun hours and 400 W panels, needs roughly 19 panels. Enter your own numbers to see yours.
What are peak sun hours?
Peak sun hours are the hours per day when sunlight is as strong as 1 kW per square meter. They are not the same as daylight hours. Most places get between 3 and 7. A solar installer or an online solar map can give the figure for your area.
Why are there system losses?
Not all the energy the panels make reaches your outlets. Some is lost in the cables and the inverter, to dust, shade, and heat. The PVWatts tool from the National Laboratory of the Rockies (NLR, formerly NREL) uses a default of 14% for these losses; the calculator starts at a more cautious 20%, and you can change it.
Is this enough to decide to buy solar panels?
No. It is a rough estimate to get an idea of the size and cost. The roof direction and angle, shade, local rules, and the equipment change the result a lot. Ask a solar installer for a site visit and a quote.