Home batteries explained: storing power, shifting cost
A home battery is the most misunderstood part of a low-carbon home. It does not generate energy or cut how much you use; it shifts when you use it, moving cheap or self-generated power into expensive hours. Whether that pays depends entirely on the gap between the price you store at and the price you avoid.
The one-sentence version
A battery charges when electricity is cheap or free (your midday solar, or a cheap overnight rate) and discharges when it is expensive (the early-evening peak), so you buy less at the high rate. The saving is the price difference, minus a small loss for storing.
How it works
Almost all home batteries are lithium-ion, the same chemistry as a phone, scaled up and managed for safety and long life. Most UK units now use lithium iron phosphate (LFP), which is more thermally stable and longer-lived than older cobalt-based chemistries, at a small cost in energy density (size), which matters little for a fixed wall unit. A battery management system (BMS) controls charging, balancing and temperature. An inverter (sometimes shared with solar in a hybrid setup) converts between the battery's DC and your home's AC.
The numbers that define a battery
- Usable capacity (kWh). How much energy you can actually draw, which is slightly less than the nameplate capacity because the BMS keeps a reserve to protect the cells. 5 to 10 kWh suits most homes; size to your evening use, not to your roof.
- Power (kW). How fast it can charge or discharge. A battery with plenty of stored energy but low power output cannot cover a big simultaneous load.
- Round-trip efficiency. About 90%. Put 10 kWh in, get roughly 9 kWh back; the rest is lost as heat in conversion. That loss is why a battery only pays when the price gap is wide enough.
- Cycles and warranty. Warranties typically run around 10 years or a set number of full cycles (often several thousand), with a guaranteed remaining capacity (often 70 to 80%) at the end.
When a battery pays, and when it does not
The case is strongest in three situations, ideally combined: you have solar and export surplus cheaply, so storing it for the evening is worth far more than exporting; you are on a time-of-use tariff with a wide gap between a cheap overnight rate and the peak; or you have an EV and can shift large loads. Where the price gap is narrow, or export rates are unusually high (so exporting beats storing), payback can run long, sometimes beyond the warranty. A battery is rarely the first thing to buy; insulation and solar usually come first.
How to size one
Work from your evening and overnight consumption, not your total or your solar size. Add up what you use between the peak hours you want to cover; that is the useful capacity. Oversizing wastes money on capacity you never cycle; undersizing leaves you importing at peak once it is empty. With solar, also consider how much midday surplus you actually have to store on an average day, which is far less in winter than summer.
AC vs DC coupling, and hybrid inverters
An expert distinction: a DC-coupled battery shares an inverter with the solar and avoids an extra conversion when storing solar, so it is slightly more efficient and tidy for new installs. An AC-coupled battery has its own inverter and is easier to add to an existing solar system or fit on its own. A "hybrid inverter" is one unit that handles both solar and battery. None is universally best; it depends on whether you are adding to existing kit or starting fresh.
Practical and safety points
Batteries attract 0% VAT until 31 March 2027, even fitted on their own. Siting matters: a cool, ventilated spot (garage, utility, outside wall) extends life, since heat is the main ageing factor besides cycling. Most home batteries do not provide blackout backup unless they include specific islanding hardware, so do not assume the lights stay on in a power cut. And a battery on a smart tariff is only as good as its control: the value comes from charging at the right hours automatically.
Where to go next
Model the saving for your home and tariff with the battery calculator, read the full payback case in are home batteries worth it, and check who benefits most in do I need a battery.
Chemistry, safety and lifespan in more depth
The shift to lithium iron phosphate (LFP) is the most important recent change in home storage. Compared with the older nickel-manganese-cobalt (NMC) chemistry, LFP is more thermally stable (far less prone to thermal runaway), lasts more charge cycles, and uses no cobalt, at the cost of being a little larger for the same capacity. For a unit bolted to a wall that hardly matters, so LFP has become the sensible default for home batteries. Lifespan is driven by two things: calendar ageing (the cells degrade slowly with time regardless of use) and cycle ageing (each full charge and discharge wears them a little). Keeping a battery cool and not routinely running it to empty both extend its life, which is why siting and the manufacturer's reserve settings matter.
What a battery will not do, and the grid-services angle
It is worth being clear about the limits. A standard home battery does not keep your lights on in a power cut unless it has specific backup (islanding) hardware and is wired for it, because for safety the inverter disconnects when the grid goes down. It does not increase how much solar you generate, only how much of it you keep. And it cannot beat the maths: if the gap between your cheap and expensive rates is small, the roughly 10% round-trip loss eats much of the benefit. On the upside, some suppliers now pay to use a share of your battery's capacity to help balance the grid, sharing the revenue with you. These "vehicle-to-grid"-style and battery grid-services schemes can improve the economics, but read the terms: they cycle your battery more, which trades a faster payback for a little extra wear.