Home Battery Storage: How to Judge Whether One Fits Your Home

• kWh — stored energy. How much electricity the battery can hold.
• kW — power. How fast it can charge or discharge, which decides which appliances it can run at the same time.

A battery with plenty of kWh but low kW can run a fridge for a long time but may not cover an oven and a kettle together. Both matter, separately. If a product lists capacity in amp-hours, convert with the amp-hours to watt-hours converter using the battery's nominal voltage.

Datasheets can list a nominal (total) capacity and a usable capacity. The battery management system keeps some energy unused to protect the cells, so usable capacity is what you can actually draw. Depth of discharge describes the same idea as a percentage. Compare products on usable kWh, from the datasheet, at the conditions stated.

• Round-trip efficiency — the share of energy put in that comes back out. Energy delivered = energy stored × round-trip efficiency. Use the figure on the datasheet and check whether it includes the inverter.
• Inverter power — a battery behind a smaller inverter can only deliver that inverter's kW. In some systems the battery shares an inverter with solar panels.
• Charge and discharge rate — maximum continuous and peak kW, which may differ.

These limits decide how much of a cheap-rate window or a solar surplus a battery can actually capture.

Capacity falls with use and age. How fast depends on battery chemistry, temperature, how deeply and how often it is cycled, and the product. There is no single lifetime that applies to all batteries. Use the manufacturer's warranty terms — typically expressed as a retained-capacity percentage after a number of years, cycles or a total energy throughput — and read which limit comes first.

Equivalent full cycles per year = energy delivered per year ÷ usable capacity

Illustrative: 5.4 kWh per day from a 9 kWh usable battery is 5.4 ÷ 9 × 365 ≈ 219 equivalent full cycles per year. Compare that with the warranty's cycle or throughput limit.

• Solar self-consumption — storing daytime surplus for the evening instead of exporting it. Value per kWh shifted ≈ import price − export payment.
• Time-of-use shifting — charging from the grid at a cheaper rate and using it at a dearer one. Value per kWh delivered ≈ peak price − off-peak price ÷ round-trip efficiency.
• Backup — powering some circuits during a power cut. Not every grid-connected battery does this: it needs backup-capable hardware and wiring, often to selected circuits only, and may limit power. Confirm this in the product's specification and with the installer.

These uses can compete for the same capacity on the same day.

All values below are invented to show the arithmetic — they are not market prices or typical figures. Battery: 9 kWh usable, 5 kW inverter, round-trip efficiency 90%.

1. Daily electricity profile — how much you use in the evening and overnight. Half-hourly smart-meter data helps; the DIY home energy audit guide shows how to find your loads.
2. Solar production profile — how much surplus you have, by season. See the solar output guide.
3. Tariff and export — import rates by time, and what you are paid for export.
4. Backup priorities — which circuits, if any, must run during a cut.
5. Size — enough usable kWh to cover the energy you can actually shift, not more.
6. Inverter limits — enough kW for the loads you want covered.

There is no ideal battery size that suits every home. For the economics of adding solar, including a battery, use the Solar Payback calculator and the solar payback guide. The Home Energy hub and Solar hub collect related tools; sources are on the methodology page.