Open menu

Solar Panel Calculator

Enter your yearly electricity use, the share you want to cover, the specific yield for your roof (kWh per kWp per year from NREL PVWatts or the EU’s PVGIS) and your panel’s rating and size. The calculator estimates the system size, number of panels and roof area. 4,800 kWh a year at a yield of 1,200 kWh/kWp needs 4 kWp, or ten 400 W panels.

Estimates only. Electrical work must be designed and carried out by a licensed electrician in line with your local wiring rules.

Measurements

Units

From your electricity bills (last 12 months).

kWh produced per kWp per year. Get it for your roof from NREL PVWatts (US) or the EU's PVGIS.

From the panel's datasheet.

From the panel's datasheet (length × width).

Results

Enter the specific yield for your location (from PVWatts or PVGIS) to size the system.

Paper and orientation
Open the list

How to use the solar panel calculator

  1. Enter your yearly electricity use from the last 12 months of bills.
  2. Choose the share of that use you want to cover.
  3. Get the specific yield for your roof from PVWatts or PVGIS and enter it.
  4. Enter your panel’s rating and area from its datasheet.

How the estimate works

Required system size = yearly use × share ÷ specific yield. Panels = ⌈required watts ÷ panel rating⌉. Installed size = panels × rating. Roof area = panels × panel area. Estimated production = installed size × specific yield.

Worked examples

Family home. 4,800 kWh a year, 100% coverage, yield 1,200 kWh/kWp: 4 kWp, ten 400 W panels, about 20 m² of panels if each is 2 m².

Half coverage. The same home covering 50%: 2 kWp, five panels.

How to interpret the results

The result is a starting point for talking to installers. Their design will account for roof layout, shading, inverter sizing and local grid rules.

Tips and common mistakes

  • Use a full year of bills so seasonal use is included.
  • Model each roof face separately in PVWatts or PVGIS if you have more than one.
  • Plan for changes such as an electric car or heat pump.
  • Check permits and grid connection rules in your area.

Getting your specific yield

In PVWatts or PVGIS, enter your address, roof tilt and orientation and a system size of 1 kW. The annual result in kWh is your specific yield in kWh per kWp. Repeat for each roof face if your panels will face different directions. Both tools also show monthly production, which helps you see how much summer output exceeds winter output.

The roof pitch calculator gives you the tilt angle to enter in PVWatts or PVGIS, and the voltage drop calculator helps you discuss cable sizing with your electrician for long runs to an inverter or outbuilding. Add the panels to your project materials list to compare quotes.

Assumptions and limitations

The estimate uses an annual average. Output varies by season and day. Electrical work must be designed and installed by licensed professionals.

Frequently asked questions

How many solar panels do I need?

System size (kWp) = yearly use × share to cover ÷ specific yield. Panels = system size in watts ÷ panel rating, rounded up.

What is specific yield?

The electricity a solar system produces per kilowatt-peak of panels per year at your location, orientation and tilt. PVWatts (US) and PVGIS (EU) calculate it for free.

Why doesn’t the calculator include a default yield?

Yield varies a lot with location, orientation, tilt and shading. A default could be badly wrong for your roof, so you enter the figure for your site.

How much roof space do I need?

Roughly the number of panels × the area of one panel, plus gaps and setbacks required by your installer and local rules.

Does this size batteries or inverters?

No. A licensed installer designs the inverter, wiring and any battery.

What does kWp mean?

Kilowatt-peak is the rated output of solar panels under standard test conditions. A system of ten 400 W panels is 4 kWp.

Will panels produce the same every month?

No. Output is higher in summer and lower in winter, and also varies with weather. PVWatts and PVGIS show monthly estimates.

Sources