Skip to content
RoomCalcs

Solar panel calculator: how many panels do I need?

Work out the solar system for your home from your electricity bill: how many panels, the size in kW, the roof space, how much it makes a year, and — if you add prices — what it saves and how soon it pays back. Add a battery for backup or off-grid.

1Your electricity use

From your bill: the kWh ("units") for a month. Use the average of a year of bills if you can — summer and winter differ a lot.

%

100 % makes as much in a year as you use. Less suits a small roof or a tight budget.

2Your location

The yearly average of full-sun hours on the panels. The Global Solar Atlas or PVGIS gives it for your address (PVWatts in the US). If you have their yearly kWh per kW instead, enter that ÷ 365 here and set the losses to 0.

%

Everything between the sunlight and your socket: wiring, inverter, dirt, shade and heat — about 15 % in cool climates and 20 % in hot ones; the sunshine choice fills it in. The 14 % default in PVWatts and PVGIS leaves out heat, which both tools work out separately.

3The panels

On the panel's datasheet. Most home panels sold in 2025–26 are about 400–450 W.

Length × width from the datasheet; about 1.95 m² (21 ft²) for a 430 W panel.

4Prices (optional)

From a quote: the total price ÷ the system size in kW. Leave empty to skip the cost and payback.

What you pay per kWh (unit), taxes included.

5Battery (optional)

Your results

Your results appear here

Type your measurements in the empty boxes. Still needed:

  • Electricity used per month (kWh)
  • Free
  • No sign-up
  • Any unit
  • PDF & CSV

How many solar panels do I need? Using the calculator

  1. 1Enter the kWh you use per month from your electricity bill — ideally the average of a year.
  2. 2Choose the sunshine where you live, or enter your own peak sun hours from PVWatts or PVGIS.
  3. 3Check the panel's power and size on its datasheet.
  4. 4Add your quote's price per kW and your electricity price to see the savings and payback, and switch on the battery for backup or off-grid.

Solar sizing formula: kW, panels and yearly output

  1. 01Yield per kW per year = peak sun hours × 365 × (1 − losses)
  2. 02System size (kW) = yearly use × share to cover ÷ yield per kW
  3. 03Panels = system size × 1,000 ÷ watts per panel, rounded up
  4. 04Yearly output = panels × watts ÷ 1,000 × yield per kW
  5. 05Payback (years) = system price ÷ yearly savings
  6. 06Battery (kWh rated) = daily use × days ÷ usable share

"Peak sun hours" are the hours of full-strength sunlight (1,000 W per m²) the panels get on an average day of the year. One kW of panels in 5 peak sun hours makes 5 kWh a day before losses — about 1,460 kWh a year after 20 % losses, close to the Global Solar Atlas figure for Lahore or Delhi. In London or Berlin (about 3.4 hours, 15 % losses) it is nearer 1,050 kWh. Divide the electricity you want to make by that yield, and you have the size of the system.

Panels come in whole units, so the system is rounded up to the next panel, and it makes slightly more than you asked for. Real output varies from year to year with the weather, and is lower in winter than in summer: a system that covers 100 % over a year still buys power from the grid in winter and exports in summer.

Solar system example

A home using 900 kWh a month (10,800 kWh a year) in a place with 5 peak sun hours gets 1,460 kWh a year from each kW of panels after 20% losses.

To cover 100% of its use it needs 18 panels of 430 W — a 7.74 kW system — making about 11,300 kWh a year on 378 ft² of roof.

Tips before you buy

  • Use a whole year of bills: one summer or winter month gives a system that is too big or too small.
  • Cut your use first — LED lights, a heat pump water heater or better insulation are often cheaper than extra panels.
  • Check the shade at different times of day and year: a chimney or tree shading part of the roof costs more output than it seems.
  • Ask each installer for the expected yearly kWh, not just the kW, and compare it with this calculator.

Frequently asked questions

How many solar panels do I need for my house?

Yearly use ÷ (peak sun hours × 365 × (1 − losses)) gives the kW; divide by the panel's watts. A home using 900 kWh a month with 5 peak sun hours and 20 % losses needs about 7.4 kW — 18 panels of 430 W. In the UK or Germany (about 3.4 hours, 15 % losses) the same home would need about 10.2 kW, or 24 panels.

How much electricity does 1 kW of solar panels make?

About peak sun hours × 365 × (1 − losses): roughly 1,050 kWh a year in the UK, Germany or the Netherlands, 1,200 in France, 1,460 in Lahore, Delhi or the eastern US, and 1,700–1,900 in Karachi, the Gulf or Arizona. A roof facing east or west, or in part shade, makes less.

How much roof space do solar panels need?

The number of panels × the size of one panel: about 1.95 m² (21 ft²) for a 430 W panel, so roughly 4.5–5 m² (50 ft²) per kW. Leave the gaps and edge distances the installer and fire rules need on top of that.

How big a battery do I need for solar?

Daily use × the days it should last ÷ the usable share. A home using 10 kWh a day that wants one day of backup needs 10 kWh usable: about 11 kWh rated in lithium (LiFePO4, ~90 % usable) or 20 kWh in lead-acid (~50 %).

How long does solar take to pay back?

The system price ÷ what it saves per year. It depends on your electricity price, how much of the power you use yourself, export payments and grants, so enter your own quote and price per kWh in the calculator.

Sources

Formulas last reviewed: September 30, 2026 ·How we check our formulas

Related tools

Guides