How to Size a Home Solar System: kWp, Panels and Batteries Explained

A solar quote is easier to judge when you know roughly what size of system you need. You can get close with three numbers: how much electricity you use, how much sun your roof gets and how much of your use you want solar to cover.

Step 1: find your electricity use

Look at your bills for a full year and add up the kilowatt-hours (kWh). Monthly use swings with the seasons, so a yearly total is better than a single bill. If you only have a few months, average them and multiply by twelve.

Planning an electric car, a heat pump or air conditioning? Add their expected use now, because adding panels later costs more per watt.

Step 2: know your sunshine

Solar output depends on peak sun hours: the average number of hours per day of full-strength sunlight on the panel. Typical values range from under 3 in northern Europe to around 6 in deserts. Free tools such as the EU's PVGIS or NREL's PVWatts give a value for your exact location, roof angle and direction.

Step 3: do the sum

A simple formula gets you most of the way:

System size (kWp) = yearly kWh to cover ÷ (peak sun hours × 365 × 0.82)

The 0.82 allows for real-world losses from heat, dust, wiring and the inverter. Example: a home using 7,200 kWh a year with 5 peak sun hours needs about 7,200 ÷ (5 × 365 × 0.82) ≈ 4.8 kWp.

Step 4: panels and roof space

Divide the system size by the panel power. With 450 W panels, 4.8 kWp means about 11 panels. A modern 450 W panel is roughly 2 m², so plan for 22 to 25 m² of usable roof without shade. Shade from trees, chimneys or neighbouring buildings can cut output sharply, so ask installers about shade analysis and optimisers.

Step 5: inverter and battery

The inverter is usually sized close to the panel power, often slightly smaller. If you want backup power or to use more of your solar at night, add a battery: multiply your average hourly use by the hours of backup you want, then allow for the usable share of the battery.

Check the quote

Try the numbers for your own home with our solar calculator, then request quotes from vetted installers to compare.