How big should my solar system be? Sizing solar from your electricity bill

Most people are asked to pick a system size before anyone has looked at how much electricity they use. We do it the other way round: your latest electricity bill shows your daily use and what you pay, and that's where sizing starts.

Step 1: your yearly use

A bill covers a billing period, usually one to three months. From the kilowatt-hours (kWh) used in that period we work out your average daily use, and from that an estimate for the year. Many bills also show the same period last year, which helps.

Step 2: enough solar to cover the year

Our Essential option is the least solar that makes as much electricity over a year as your home uses, based on the average sunshine at your address (from NASA's long-term records, and Google's roof data where it's available). We never go below a 5 kW system, which is the smallest size that usually makes sense to install.

Step 3: when you use it matters

Solar only makes electricity during the day. As a rule of thumb, a home uses about 40% of its solar as it's made (the fridge, appliances, working from home); the rest goes back to the grid for a small feed-in credit. The electricity you use after dark still comes from the grid unless you have a battery.

Step 4: with a battery, size for winter

A battery stores daytime surplus for the evening. Winter has the shortest days, so when you'd like a battery we size the panels so that even in June to August there's usually enough left over to charge it for your evening use, and the battery is the smallest size that holds that evening use.

• Essential: solar only, sized to your yearly use.
• Recommended: solar and a battery, sized for winter evenings.
• Maximum: the same solar and battery, plus a smart EV charger.

Step 5: checked against your roof

Where Google's Solar API has data for your home, we check how much usable roof there is and never recommend more panels than it suggests will fit. The final design, including how many panels fit and where, is worked out before your 15-minute call and shown to you on it.

Every figure is an estimate from your bill, your address and typical patterns. Your real savings depend on your usage, the weather and your electricity plan.