Battery Storage Cost: What Affects the Price

Usable capacity versus nominal capacity

A battery's nominal capacity is the energy its cells contain. Its usable capacity is what the battery management system will release, after the reserve it holds back to keep the cells within their safe operating window and to protect their life. The gap between the two figures differs between products and chemistries. When a quotation states a capacity, establish which figure it is quoting; when comparing two products, compare usable against usable. Our battery sizing page explains how usable capacity is matched to a household's evening demand.

Continuous and peak power output

Capacity in kWh tells you how much the battery holds. Power output in kW tells you how fast it can deliver it. A store with generous capacity and modest output will cover lighting, a fridge and a television comfortably and will not, on its own, cover an electric oven, a kettle and an immersion heater at the same time — the shortfall simply comes from the grid. Peak output is a short-term rating above the continuous figure, for motor start-up and other brief surges, and it matters most where the battery has to run circuits during an outage. Both figures belong in the specification you are quoted.

Chemistry

Chemistry determines cycle life, thermal behaviour, energy density, the temperature range within which a unit may be sited, and cost per usable kilowatt-hour. Lithium iron phosphate is widely used in domestic storage for its cycle life and thermal stability, at somewhat lower energy density than other lithium chemistries, which can mean a physically larger unit for the same capacity. That trade-off is worth understanding before deciding where a battery will live. See LFP batteries and our general page on battery storage technology.

AC coupling, DC coupling and the hybrid inverter question

A DC-coupled battery connects on the array side of a hybrid inverter, so solar energy goes into the battery without an extra conversion stage. It is efficient and tidy, and it means the inverter has to be a hybrid unit. An AC-coupled battery has its own inverter and connects on the mains side, which lets it be added to an existing array without touching the original inverter, and lets it charge from the grid on a cheap overnight rate. It involves an additional conversion stage and additional hardware. Neither is universally better; the right answer depends on whether the array already exists, the condition and age of the existing inverter, and what you want the battery to do. If your array is being installed now and storage is a possibility later, specifying a hybrid inverter at the outset is normally cheaper than replacing a solar-only inverter afterwards.

Backup and EPS functionality

Grid-connected inverters disconnect during a power cut so that they cannot energise a network someone may be working on. Keeping selected circuits alive through an outage therefore requires equipment designed for it: an inverter with an emergency power supply capability, a changeover arrangement, in most cases a separate backup distribution board, the cabling to bring the chosen circuits onto it, and labelling. It also requires a decision about which circuits are worth keeping — heating controls, a fridge, lighting and sockets in one room are a common answer; whole-house backup is a different scale of installation. This is one of the larger discretionary items on a storage quotation.

Siting, cable run and electrical work

Manufacturers specify an operating temperature range, ventilation and clearance requirements, and mounting conditions. A garage, plant room or utility area usually suits a battery better than a loft, which is hot in summer and cold in winter. There may be restrictions on siting near escape routes, and there must be room for a service visit or a replacement later. Weight matters for wall mounting and for some floors. The distance from the battery to the inverter and consumer unit sets the cable size, the containment and the labour. On the electrical side the board needs a spare way, a suitable protective arrangement, and main earthing and bonding that meet current requirements. All of this is established at survey.

A battery does not generate anything. It moves energy from a moment when it is cheap to a moment when it would otherwise be expensive, and it loses a little in the process through round-trip efficiency. Its value is therefore the price difference it exploits, multiplied by the number of times it exploits it, less the losses. Two things follow from that, and they matter more than the capacity figure.

Cycling frequency. A battery that is filled and emptied on most days does many times the work of one that is half-filled twice a week. Capacity beyond what you can reliably fill and reliably empty adds cost without adding cycles. This is why oversizing storage tends to disappoint: the extra kilowatt-hours sit idle for much of the year. It is also why a summer-only assessment is misleading — in winter a solar array in the UK may not fill a battery at all, and whether it still earns its keep then depends entirely on the second factor.

Tariff. On a flat-rate tariff the only price difference available is between your own generation and the grid import price, so the battery is limited to storing surplus solar. On a time-of-use tariff with a wide spread between the cheapest overnight rate and the daytime or peak rate, the battery can also charge from the grid and displace expensive import all year round, including in the months when the array contributes little. The same battery in the same house can therefore have quite different value depending only on the supply contract, and tariffs change. Any figure you are shown for battery savings is a figure about a tariff as much as it is about a battery.

There is a third consideration that is easy to miss: a battery changes what happens to your surplus. Energy stored and used at home is import you avoid paying for; energy exported is income at whatever rate your Smart Export Guarantee supplier pays. Whether storing is worth more than exporting depends on the gap between your import price and your export rate. Our payback page works through that distinction, and our advice article on the Smart Export Guarantee explains how export rates are set.

Electrical storage batteries were added to the list of qualifying energy-saving materials from 1 February 2024. Installation of qualifying energy-saving materials in residential accommodation in Great Britain is zero-rated for VAT from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027. That applies to a battery installed alongside solar and to a battery installed on its own, subject to the conditions in the notice. Commercial premises and materials supplied without installation are treated differently. HMRC VAT Notice 708/6 is cited in the sources below; our advice article on VAT on solar panels covers the same ground in more detail.