What size battery do I need?

Battery size follows your evening and overnight electricity use, not your array size and not your house size. The useful starting point is the energy you consume between the array shutting down and starting again — typically from late afternoon through to the following morning. A battery sized around that figure gets emptied and refilled regularly, which is what makes it worth having. Capacity beyond it sits unused for most of the year. Two other numbers matter alongside capacity: the power rating in kW, and whether you are on a tariff with cheap overnight import.

Capacity: the kWh number

Pull your consumption apart by time of day. Smart meter data through your supplier’s app is usually enough. What you are looking for is the evening-to-morning block, because that is what a battery is actually asked to cover.

Two households with identical annual consumption can need very different storage. One cooks electrically, runs a tumble dryer at night and charges a car on the drive. The other has gas heating and hot water, and its overnight load is a fridge and a router. The first has somewhere to put stored energy; the second does not.

Seasonality complicates it. In summer the array will fill a battery easily and the constraint is how much you can use. In winter there may not be enough generation to fill it at all, so the battery’s job shifts to moving cheap overnight import into expensive daytime hours — if your tariff supports that.

Power: the kW number that people miss

Capacity tells you how much energy is stored. The power rating tells you how quickly it can come out.

If the battery can discharge at a modest rate and the house pulls more than that, the shortfall comes from the grid regardless of how full the battery is. Kettles, ovens, showers, induction hobs and EV chargers all push past modest discharge limits. It is a common source of disappointment: a battery with plenty left in it, and the meter still showing import.

The same applies to charging. A large battery with a limited charge rate may not fill during a short winter window, or within an off-peak tariff period.

How the array size interacts

A battery only earns its keep if something fills it. Pairing a large battery with a small array on a shaded roof means relying on grid charging, which is a reasonable strategy on the right tariff but a poor one on a flat rate.

Conversely, a generous array with no storage on a house that is empty during the day exports the majority of what it produces. Export is paid under the Smart Export Guarantee at a rate the supplier sets, and it is generally worth less than the import you avoid by using the energy yourself. Storage is the mechanism that converts export into avoided import.

Tariffs change the answer

Time-of-use tariffs with a cheap overnight window make batteries useful year-round rather than seasonally, because there is always something to charge from. They also reward extra capacity more than a flat tariff does, since every stored unit displaces an expensive one.

If you are on a flat-rate tariff and have no intention of moving, size the battery to summer surplus and evening use, and do not buy capacity on the assumption of a tariff you have not signed up to.

Physical and practical limits

Storage has to live somewhere with sensible temperatures, ventilation and access for servicing: garages, utility rooms and plant cupboards are the usual candidates. Wall-mounted units need a suitable substrate. Floor-standing units need floor space that stays clear.

There is also the inverter. A hybrid inverter handles solar and storage together and is the cleaner route when both are installed at once. Retrofitting storage to an existing string inverter usually means an AC-coupled battery inverter instead, and the choice affects both what capacity is sensible and how the system behaves in a power cut.

Getting the answer for your house

We look at half-hourly consumption data where you have it, your tariff, your array or proposed array, and the space available. That produces a capacity and a power rating that suit the way your household actually runs.

Illustrative capacity ranges published by independent bodies are industry figures for a typical home, not Solar Fx figures for yours. Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.