The battery is usually the single most expensive part of a solar and battery installation, and the right size depends on one thing: your average daytime electricity usage once overnight and EV charging are stripped out. As a rule, undersize if you're unsure how your usage will change, since storage can be added later but never removed. Oversize only for a genuine power cut reserve or a confirmed future load like a heat pump.
Battery sizing is often approached as a summer versus winter question: charge from solar in summer, use it in winter. That logic only applies if you are trying to go fully off grid. For most UK homeowners chasing the best return on investment, a battery works on a daily cycle, not a seasonal one. It fills overnight or from solar, then discharges through the following day. Sizing it around anything longer than that produces an oversized, underused battery.
Find your annual electricity usage - Start with your bill or online account and find the figure for estimated annual usage, measured in kWh. This is your baseline before any of the adjustments below.
Work out your overnight usage - Once a battery is installed, you will typically move to a time of use tariff with a cheap overnight rate and a more expensive daytime rate. As of August 2026, Intelligent Octopus Go's overnight rate sits at around 7p per kWh against a standard variable day rate of roughly 26p per kWh, a gap wide enough that almost every home with a battery benefits from shifting usage into the cheap window.
A battery cannot charge and discharge at the same time, so it is usually charging overnight rather than covering your overnight load. That means overnight usage, and any electric vehicle charging done overnight, should be stripped out of the figure you are sizing the battery against. Miss this step and you end up with a battery sized to cover electricity it was never going to discharge to anyway.
Calculate your average daily load - Take your total annual usage, subtract your overnight usage, and divide the result by 365. That gives you your average daytime electricity consumption, the core number the rest of the calculation is built on.
Adjust for how your usage will change - A home battery is a 10 to 15 year investment, so it is worth sizing it for where your usage is heading, not just where it is today. A heat pump, an extension, or a growing family will push daytime usage up. An electric AGA being decommissioned, for example, can remove a significant daytime load. One common adjustment homeowners get wrong is planning for an EV: since EV charging is almost always scheduled into the cheap overnight window, an EV does not usually change your daytime battery sizing at all.
If you are unsure how usage will move, size down rather than up. Storage can be added later. Money spent on unused capacity today cannot be recovered.
Decide on a power cut reserve - Some batteries, including Tesla Powerwall 3 and Sigenergy SigenStor, let you hold back a percentage of capacity in reserve for a power cut. Most homeowners reserve around 10% of total capacity for this. If you experience frequent outages, you may want to hold back more.
Add it up for your target capacity - Add your adjusted daytime load from Step 4 to your power cut reserve from Step 5. That total is your target battery capacity, the number to compare against when you start getting quotes.
Comparing the wrong capacity figure - Manufacturers quote a nominal capacity and a depth of discharge, and the two are not the same number. A battery listed as 10kWh with a 95% depth of discharge gives you 9.5kWh of usable capacity. Some manufacturers, including Tesla, quote the usable figure directly (the Powerwall's 13.5kWh is a usable figure against a nominal capacity closer to 15kWh). Always compare usable against usable.
Ignoring charge rate - If you are relying on an overnight off-peak window, your battery needs to fully charge inside it. Charge rate and inverter rate are different specifications: the inverter rate governs how fast the battery can power your home, the charge rate governs how fast it fills up. A single Powerwall 3 takes around two hours 42 minutes to charge fully from empty; stack two together and that rises to roughly three hours 22 minutes, because the charge rate only scales up to a point. Check this against your actual off-peak window length before buying.
Pairing a large battery with a small inverter - This is mostly a risk with non all-in-one systems built from separate components. A 20kWh battery behind a 3.68kW inverter can only feed 3.68kW into the house at any one time, which may not cover peak demand when several appliances are running at once.
Sizing around export income rather than your own usage - Batteries earn their return primarily by shifting your own usage from expensive daytime rates to cheap overnight rates, not by exporting to the grid. Export rates do add income, but they move: Octopus cut its Outgoing Octopus flat export rate from 15p to 12p per kWh in March 2026, and rates across the market have followed a similar direction. A 10 to 15 year sizing decision should not be built around a rate that can change with 30 days' notice.
Work out your daytime load, strip out overnight and EV usage, adjust for the next decade, add your reserve, and check the four common quoting traps before you commit. That sequence, done properly, is what stops a battery being oversized, undersized, or bought against the wrong number entirely.