How many solar panels do I need? System size, roof space and battery

Short answer: take the kWh you use in a year and divide it by what 1 kW of panels makes where you live — about 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 home using 900 kWh a month with 5 peak sun hours needs about 7.4 kW: 18 panels of 430 W on roughly 35 m² (378 ft²) of roof.

Step 1: your yearly electricity use

Look at your bills for the kWh (on many bills called "units") used in a month. Add up a whole year if you can, or multiply one month by 12 — but summer and winter can differ by half, so one month alone is a rough guess. The average US home uses about 900 kWh a month, 10,800 kWh a year (US EIA). A family home in the UK or Germany uses nearer 3,000–4,500 kWh a year.

If you plan a heat pump or an electric car, add their use now. Adding panels later costs more than fitting them at the start.

Step 2: what 1 kW of panels makes where you live

Solar output depends on peak sun hours: 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. Then come the system losses: wiring, dirt, shade, the inverter and heat. Panels lose output when they get hot, so the losses are about 15 % in a cool climate and 20 % or more in a hot one.

These match the yearly yields in the World Bank's Global Solar Atlas for well-placed panels: London 1,027, Berlin 1,071, Paris 1,140, New York 1,414, Lahore and Delhi 1,466, Karachi 1,697 and Phoenix 1,897 kWh per kW. For your exact roof — its tilt, direction and shade — enter your address in the Global Solar Atlas, PVGIS (Europe, Africa, Asia) or PVWatts (US). If they give you the yearly kWh per kW, enter that ÷ 365 as the sun hours in the solar panel calculator and set the losses to 0.

Step 3: system size and number of panels

• System size (kW) = yearly use ÷ kWh per kW
• Panels = system size × 1,000 ÷ watts per panel, rounded up

Most home panels sold in 2025–26 are about 400–450 W. With 430 W panels, the calculator gives:

The kW is what you need exactly; the panel count is rounded up, so the system makes a little more.

Step 4: will it fit on the roof?

A 430 W panel is about 1.95 m² (21 ft²) — for example 1,722 × 1,134 mm — so allow roughly 4.5–5 m² (50 ft²) per kW of panels, plus the gaps and edge distances the installer and fire rules need. Panels make the most facing the equator (south in the northern hemisphere) at a tilt close to your latitude. East–west roofs make roughly 10–20 % less, which you can make up with a few extra panels. Not sure of your roof's angle? The roof pitch calculator works it out from the rise and run, and the roofing calculator gives the roof area.

Battery: how many kWh?

A battery stores daytime power for the evening, or runs a cabin or home off the grid:

• Usable kWh = daily use × the days it should last without sun
• Battery to buy (rated kWh) = usable kWh ÷ the usable share — about 90 % or more for lithium iron phosphate (LiFePO4), only 50 % for lead-acid

An off-grid system also needs enough panels to refill the battery in winter, a charge controller and an inverter big enough for everything that runs at once — size those with the installer.

Cost and payback

Payback in years = system price ÷ what it saves a year. Some recent published prices, before any grants:

Your own quote is what counts: divide its total by the system size to get the price per kW, and enter it with your electricity price in the calculator. Three things change the answer a lot:

• How much you use at home. Exported power is usually paid far less than the price you buy at — in France, new contracts since June 2026 pay 1.1 c€ per kWh exported; the Netherlands ends net metering on 1 January 2027; Pakistan moved new users from net metering to net billing in February 2026.
• Tax credits. In the US, the 30 % federal residential clean energy credit (25D) ended for systems installed after 31 December 2025 (IRS).
• Grants and export payments where you live, which change often — check the current rules before you sign.

Ask every installer for the expected yearly kWh, not only the kW, and compare at least three quotes.

Frequently asked questions

How many solar panels do I need to power a house?

Divide the yearly kWh by the yield per kW where you live, then by the panel's watts. A typical US home using 900 kWh a month with 5 peak sun hours needs about 7.4 kW, which is 18 panels of 430 W. In the UK or Germany, a home using 4,200 kWh a year needs about 4 kW, or 10 panels.

How much electricity does one solar panel make?

A 430 W panel makes its watts × peak sun hours × 365 × (1 − losses) in a year: about 450 kWh in the UK or Germany, 520 in France, 630 with 5 sun hours and 740 with 6.

How many solar panels do I need for 1,000 kWh a month?

About 8.2 kW with 5 peak sun hours (20 panels of 430 W), 9.9 kW with 4 (24 panels) and 11.4 kW in northern Europe (27 panels).

Does a 100 % system mean no electricity bill?

Not quite. It makes as much as you use over a year, but most of it in summer. In winter you still buy power from the grid, and exported power is often paid less than what you buy, so a fixed charge and some winter use usually remain.

Do solar panels work on cloudy days?

Yes, at a lower output. The yearly sun hours already include cloudy days, which is why a UK or German roof makes about 1,050 kWh per kW a year and a desert roof nearly twice as much.