Is a battery worth it for your home?

We compare a full year of electricity costs (electricity bought, standing charge, minus export payments; not other energy or FIT generation payments) for your home on its current tariff, the best of the tariff switches we model without buying anything, a smaller and a larger battery, and your quote if you give one. A battery is credited only with what it adds beyond that switch. This is not a market comparison: the switches we model are a 12p tied export rate, Octopus Go (with an electric car) and one time-of-use tariff you enter. Each depends on terms we cannot confirm for you, such as the import price you would pay with that supplier or an exit fee from your current tariff, and the result says when the answer depends on one.

• Solar output: PVGIS 5.3 averages for five UK places (London, Cardiff, Manchester, Belfast, Edinburgh), 14 per cent system losses, for the roof direction you choose.
• How much solar your home uses, with and without a battery: a simplified Worthwatt model of representative dull, average and bright days in each month, fitted to examples MCS publishes in MCS 032:2025. Across the 57 no-battery examples we fitted (all for a home using 3,500 kWh a year) it is within 2.5 per cent on average (at most 6.7 per cent). Those figures describe the examples, not a measured accuracy for your home's savings.
• What a battery adds: against MCS's three published battery examples, the model's extra saving was 1 to 18 per cent lower than MCS's, so the hopeful case adds 20 per cent to the part of the saving that comes from storing solar. Those examples were also used to fit the model, so they cannot show how far it is out for other homes, in either direction.
• Battery: charges from spare solar, and from cheap overnight electricity on a time-of-use tariff, sharing the same capacity (never counted twice); round-trip efficiency 90 per cent (85 per cent in the cautious case). Overnight charging is limited by the battery's charge power and the cheap hours: your quote's figure, or the lowest maximum charge power among the battery datasheets we hold for that size (Sigenergy SigenStor: 2.5 kW for a 5.2 kWh usable battery, 4.6 kW for 8.76 kWh). Charging from solar and discharging are not limited by power in the model.
• In the hopeful case the battery is filled overnight as if the next day's sunshine were known; in the cautious case, for an average day of the month, and electricity bought for it that the real day does not need is counted as wasted. In our checks that difference was up to 13 per cent of the saving.
• Overnight use: we assume 10 per cent of a home's electricity falls in a 5-hour cheap window (our assumption, scaled for other window lengths).
• Battery prices without a quote: DESNZ 2025-26 median installed price per kWh of nominal capacity, MCS-certified batteries in existing UK homes, usable capacity taken as 90 per cent of nominal.
• The range: a cautious case (85 per cent efficiency, more day-to-day variation in sunshine, a 10-year life, and capacity falling in a straight line to 70 per cent after 10 years, the lowest that the Enphase and Fox ESS UK warranties guarantee (Fox ESS also sets a throughput limit; Tesla guarantees 80 per cent)) and a hopeful case (90 per cent, less variation, 12 years, plus the 20 per cent above on stored solar). How much more or less variation is our own choice for showing the spread, not a published figure. Prices are assumed not to change. The straight-line fall in capacity is our simplification of the warranty figure, which is a guaranteed minimum, not a forecast.
• Not included: savings on electric car charging or heating, backup value, exit fees, and Feed-in Tariff generation payments (unchanged).
• If you are not sure of an answer that matters, such as your Feed-in Tariff or whether you have a smart meter, we work it out each way and show the more cautious result; if the answer would change, we say what to check instead of giving a verdict.
• Outside the range MCS's method covers (yearly use under 1,500 or over 6,000 kWh, solar generation under 1,500 or of 6,000 kWh or more, an existing battery, a heat pump or electric heating), or for a battery outside 1 to 30 kWh, we say so instead of giving a verdict.

MCS has not reviewed or endorsed this planner or Worthwatt. MCS 032 is MCS copyright; we use our own model and do not reproduce its tables.

A simpler version worked by hand, with every source (it leaves out capacity loss and overnight charging): is a solar battery worth it.