What size battery do I need?

Start with your evening and overnight use

A battery is most useful for the electricity you use when your panels are not producing — evenings, overnight and early mornings. Your smart meter app, or your supplier's online account, will usually show your usage by the half hour.

As a rough guide, Ofgem's typical figure for a medium-use home is about 2,700 kWh of electricity a year — around 7.4 kWh a day. Homes with an electric car, a heat pump or electric heating use considerably more.

Nominal vs usable capacity

Battery datasheets often quote two figures. Nominal capacity is the total energy the cells hold; usable capacity is what you can actually draw, because a small reserve is kept to protect the battery. A SOFAR BTS 5K module is 5.12 kWh nominal and 4.6 kWh usable. Compare batteries on usable capacity.

Things that push the size up

• Charging overnight on a time-of-use tariff and running the house from the battery all day
• An electric car or heat pump
• A large solar array that regularly produces more than you can use
• Wanting backup power to last through a longer outage

Why modular helps

With a modular system like PowerAll, you can start with fewer battery modules and add more later — from one module (5.12 kWh) up to six (30.72 kWh). New modules can be mixed with existing ones, so you are not locked into today's guess.

The battery also has to suit the inverter and your grid connection, so the final size is best confirmed by an installer who has looked at your usage, roof and supply.