Home battery storage: sizing, time-of-use tariffs and the VAT position

Adding a battery to a solar PV system changes the economics of the whole setup, because it lets a household store daytime surplus for use later rather than exporting most of it at a typically lower rate, or draw on cheap grid electricity overnight instead of paying peak-time prices. Getting the size and tariff right matters more than most people initially assume.

Domestic batteries commonly range from around 5kWh up to 16kWh or more, and the right size depends on the household's evening and overnight electricity use, the size of the solar system feeding it, and whether the main goal is maximising self-consumption of solar generation, exploiting a cheap overnight import tariff, or both. As an illustration of typical UK package sizing in 2026, a small system might pair 2.7kWp of panels with a 10kWh battery, a mid-sized and commonly cited "most popular" configuration pairs around 5-6kWp with a 16.1kWh battery, and larger installations on bigger homes have paired 10kWp of panels with 16-32kWh of storage. These are illustrative rather than a recommendation for any specific household — an installer sizing a system properly should be looking at your actual half-hourly usage data, not just headline package sizes.

Time-of-use tariffs are what turn a battery from a solar-only tool into something that can also save money independently of how sunny it's been. Two broad types are common in the UK market. Flat overnight tariffs offer a fixed, usually cheap, rate for electricity used within a set window, commonly a few hours overnight, and are relatively simple to use since the battery just needs to be charged within that window, whether from the grid, solar, or both. Half-hourly, wholesale-linked tariffs go further, with prices that can swing across the day between very cheap or even negative rates at times of high renewable generation and much higher rates at peak demand. Exploiting this kind of tariff properly generally requires a battery and inverter set up to respond automatically, since manually watching and reacting to 48 price changes a day isn't realistic for most households — compatible systems typically use either the battery manufacturer's own app-based automation or a hybrid inverter capable of following a time-of-use price signal.

On cost, the picture for a battery-only or combined solar-and-battery installation is currently favourable on VAT. Installing battery storage in qualifying residential accommodation in Great Britain is zero-rated for VAT under HMRC's temporary relief (VAT Notice 708/6), and this applies whether the battery is fitted alongside new solar panels or, since 1 February 2024, retrofitted as a standalone battery to an existing system with no new panels involved. The relief runs until 31 March 2027, reverting to 5% from 1 April 2027, and is applied automatically by a VAT-registered installer with no separate claim process. As with solar panels, it's worth knowing this covers new qualifying installation work rather than standalone repairs — replacing a faulty battery module on an existing system as a like-for-like repair, for instance, would not automatically qualify for the same relief.

Support outside VAT differs by nation, and this is somewhere generic UK-wide advice can mislead. In Scotland, Home Energy Scotland previously offered interest-free loan and grant funding covering solar panels and home battery storage, but that funding for standalone solar and battery installations was withdrawn from around June 2024. Some funding can still be available where solar or battery storage forms part of a wider package alongside a qualifying heat pump installation, and Warmer Homes Scotland can fund solar as part of a broader home energy upgrade for households that meet its income or benefits-based eligibility criteria. Anyone in Scotland considering a battery should check current eligibility directly with Home Energy Scotland rather than assume rest-of-UK grant terms apply, since the position has moved and may move again.

Before committing to a specific battery size, it's worth asking a prospective installer to model your likely usage pattern against both a simple flat overnight tariff and, if you're open to it, a half-hourly dynamic tariff, since the "right" battery size and the tariff you end up choosing are genuinely linked decisions rather than separate ones made in isolation.

It's also worth asking a few practical installation questions that get less attention than sizing and tariffs but affect day-to-day performance. Where will the battery physically sit — garage, utility room or outside wall — and does that location suit the manufacturer's stated operating temperature range, since batteries kept somewhere very cold or very hot can perform and age differently to one kept in a more stable indoor location. Is the battery compatible with your existing or planned solar inverter, or does it need its own separate inverter, which affects both cost and how neatly the whole system integrates. And does the installer's proposed setup let you monitor solar generation, battery charge level and grid import/export together in one app, rather than as three separate systems that don't talk to each other — this makes it far easier to spot afterwards whether the tariff and battery combination is actually delivering the savings that were modelled at quote stage.

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