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About the solar panel calculator
How many panels do you need to cover your electricity? It depends on how much electricity you use, how much sun your roof gets, and how strong each panel is. This calculator does the math.
Type your electricity use per month (in kWh, from your bill), your price per kWh, and the sunshine where you live. Choose the panel power. You see the system size, the number of panels, the energy it makes in a year, the savings in the first year, the cost, the payback time, and the roof area needed.
Open More options to add a subsidy or tax credit, the installed price per watt, a yearly electricity price rise, and the efficiency of your system.
How to use it
- Type the electricity you use per month in kWh — see your bill
- Type the electricity price per kWh
- Choose the sunshine where you live, from little to desert
- Choose the panel power — 400 W is common today
- Optional: change how much of your electricity you want to cover (100 % is all of it)
- Open More options for price, subsidy, price rise and losses
How it is worked out
- Daily need = monthly kWh × 12 ÷ 365
- System size (kW) = daily need ÷ (sun hours × system efficiency)
- Panels = system size ÷ panel power, rounded up
- Yearly output = system size × sun hours × 365 × efficiency
- Savings = kWh used from the panels × price per kWh, with a small yearly price rise and a small yearly loss of panel power
Sun hours here are "peak sun hours" — hours of full-strength sunlight a day, as an average over the year. It is not the number of daylight hours. System efficiency is usually 75–85 %, because of heat, wiring, dirt and the inverter.
Example
A home that uses 900 kWh a month, at $0.17 per kWh, with 4.5 sun hours, 400 W panels, an efficiency of 80 % and an installed price of $2.80 per watt:
- System size: 8.4 kW — 21 panels
- Output: about 11,038 kWh a year
- Savings in the first year: about $1,836
- Cost: about $23,520 before any subsidy
- Payback: about 11.6 years
- Net gain after 25 years: about $32,700
- Roof area: about 40 m² (429 sq ft)
Panels by electricity use
At 4.5 sun hours, 400 W panels and 80 % efficiency:
| Use per month | System size | Panels |
|---|---|---|
| 300 kWh | 2.7 kW | 7 |
| 600 kWh | 5.5 kW | 14 |
| 900 kWh | 8.2 kW | 21 |
| 1,200 kWh | 11.0 kW | 28 |
| 1,500 kWh | 13.7 kW | 35 |
What changes the result
- Sun: a sunnier area needs fewer panels for the same energy
- Roof direction and shade: in the northern hemisphere a roof facing south is best. Shade can cut output a lot
- Price of electricity: the higher it is, the faster the system pays back
- Subsidies and tax credits: these change from country to country and from year to year. Type the percent you can get under Subsidy or tax credit
- Battery: a battery adds cost but saves more of the evening use. This calculator does not include one
Frequently asked questions
How many solar panels do I need?
Take your yearly kWh, divide by (sun hours × 365 × efficiency × panel kW). The calculator does it and rounds up.
How much does a solar system cost?
It depends on the country and the installer. The calculator uses a price per installed watt. Check it against quotes in your area.
How long does a solar system last?
Panels often last 25–30 years and lose about 0.5 % of output per year. An inverter may need to be replaced once, after about 10–15 years.
What if I sell power back to the grid?
This calculator values all the power you use yourself at your price per kWh, up to your yearly use, and does not count extra power sold. Rules for selling are different everywhere.
What are peak sun hours?
The number of hours per day the sun would give 1,000 W per m² — the strength panels are rated at. It is a smoothed yearly average for your area.
Is this a quote?
No. It is a planning estimate. Ask installers for a quote.
Good to know
- Works with any currency — choose it at the top
- Your numbers are saved only in this browser
- Nothing you type is sent anywhere
Related tools
- Electricity Cost Calculator — what your devices cost
- Mortgage Calculator — see financing a home
- Compound Interest Calculator — compare with investing the money
- Inflation Calculator — what future savings are worth
- Area Converter — convert m² and sq ft
