Battery Capacity: Nominal kWh, Usable kWh and Depth of Discharge

A battery sold as 10kWh will not deliver 10kWh to your home. The headline figure on a datasheet or quote is the nominal capacity, the total energy the cells hold. What you can actually draw is the usable capacity, and it is always smaller, because the system holds some energy back to protect the battery's life. For most UK-installed solar batteries, only 90 to 95 percent of the total is available for use¹.

The industry's own calculation makes the gap explicit. Both the MCS battery standard and the MCS guide used for solar self-consumption assessments state the same formula: usable capacity equals nominal capacity multiplied by the maximum depth of discharge². Usable capacity is defined as the energy within the storage device available to the customer for any domestic storage application, including solar self-consumption². On top of that, many systems are set to stop discharging at a reserve level, often around 20 percent of total capacity, which cuts the figure further⁴.

Understanding this distinction matters when comparing quotes, because two batteries with the same headline kWh can deliver different amounts of electricity depending on their chemistry, their depth of discharge limit and how the reserve is set. It also matters when working out what a battery will actually cover each evening, which is the question most households are really asking.

Rated versus usable energy: why the two figures differ

Nominal capacity is the manufacturer's measure of everything the battery holds when full. Usable capacity is what remains after the system's own protection limits are applied. The two figures are related by a single rule, used across the MCS documentation that governs how installers must assess battery benefit: "Usable Capacity = Nominal Capacity × Maximum Depth of Discharge"². Where a datasheet does not state usable capacity clearly, this is the calculation installers are told to apply³.

The definition behind the formula is worth reading in full, because it sets what the number is for. The MCS standard defines usable capacity as "the energy within the storage device available to the customer for use of any domestic energy storage application, including solar PV self-consumption"². In other words, it is the energy your home can draw, not a laboratory measure of the cells.

For a modern lithium battery installed in the UK, that protection limit typically leaves 90 to 95 percent of the total available¹. A 10 kWh nominal battery therefore delivers roughly 9 to 9.5 kWh at most, before any reserve setting is applied. The gap is not a defect or a marketing trick: discharging a lithium cell completely, every cycle, shortens its life, so the system holds a margin back by design. The size of that margin varies with chemistry, which is covered in more detail on home battery chemistries.

When comparing products or quotes, the practical rule is to ask which figure is being quoted. A quote built on nominal capacity will look larger than one built on usable capacity for the same physical battery. The MCS self-consumption calculation that installers use for grant and export purposes works from usable capacity, so that is the figure that determines what the battery is assessed as doing for your home³.

How much stored power is genuinely available to a household

Depth of discharge, usually written as DoD, is the fraction of the battery's total energy that has been drawn off, expressed as a percentage. A battery that is half empty is at 50 percent depth of discharge. The maximum depth of discharge is the limit the system will allow before it stops supplying the house, and it is the number that converts nominal capacity into usable capacity through the formula above².

For lithium batteries installed in UK homes, most systems let you use 90 to 95 percent of the total, to protect battery life¹. That is a deliberate engineering choice: the last few percent of a lithium cell's charge is the most damaging to extract, so the system refuses to go there.

Lead acid batteries are a different matter. The MCS standard states plainly that "the depth of discharge must be 50% for lead acid batteries"². That means a lead acid bank needs to be roughly twice the nominal size of a lithium battery to deliver the same usable energy, which is one reason lead acid has largely disappeared from mainstream home installations, though it remains in use for off-grid setups, as covered on lead-acid, AGM and gel batteries.

Depth of discharge limits also appear in grid service arrangements. In Nesta's Dynamic Demand challenge, one finalist, Upside, imposed a maximum depth of discharge of 20 percent on its equipment "to protect battery life"⁵. That is an extreme case, from a commercial flexibility product rather than a home installation, but it shows the same principle at work: whoever sets the operating rules trades available energy against battery longevity.

Setting a minimum level the battery will not go below

Separate from the maximum depth of discharge is the reserve setting, which is a floor the household or installer chooses. Energy Saving Trust describes how this works in practice: "When the battery gets down to a set level, maybe 20% of the total storage capacity, your system will stop taking electricity from the grid or your panels to supply your home"⁴. The same guidance, in its England-specific version, gives the same figure of around 20 percent⁶.

The reserve does two jobs. First, it protects the battery in daily cycling, keeping it out of the deepest discharge range even if the household's evening use runs long. Second, and often more importantly, it can be held as an emergency store: energy kept back so that the battery has something in it if the grid goes down. A household that sets a 20 percent reserve on a 10 kWh battery is choosing to hold around 2 kWh permanently in reserve, which reduces what is available for the evening peak but means the battery is never truly empty.

The setting is adjustable in most systems, and the right level depends on what the household wants the battery for. A home using the battery purely to shift solar power from day to night might set a low reserve to maximise usable energy. A home that values backup for power cuts might set a higher one. The trade-off is simple and worth stating plainly: every percentage point held in reserve is a percentage point not available for day-to-day use.

What counts as usable, covering the 4pm to 8pm peak window

Usable capacity is the number that answers the practical question: what will this battery actually run? The Centre for Sustainable Energy notes that energy stored in a battery "can then be used during peak hours (typically 4pm to 8pm)"⁷. That evening window is when most homes draw hardest on the grid, and it is exactly the period a solar battery is designed to cover, because panels stop producing as the sun sets while household demand rises.

The scale of that demand is smaller than many people expect. The average UK household uses 1.5 to 3 kWh between sunset and bedtime, so a 5 kWh battery covers that with headroom for most homes¹. This is why usable capacity, not nominal capacity, is the right basis for sizing: a battery with 5 kWh usable will cover the evening peak for a typical household even though its nominal figure might be higher.

The same logic applies to tariff arbitrage, where the battery is charged cheaply and discharged expensively. Which? notes that "you can charge up your battery during off-peak hours, at the lowest rate, and use the energy during the peak-rate period to minimise your electricity bill"⁸. How much of the peak that displaces depends on usable capacity and on the battery's power rating, covered on battery power ratings. Stacking this with time-of-use tariffs is covered on batteries and smart tariffs.

Solar Energy UK describes the underlying pattern: "They enable power produced during the day to be stored and used at another time, for example in the evening, when demand is higher"⁹. Oxfordshire Council's retrofit guidance makes the same point for households without batteries: "you can store any unused energy for use later, for example to power your home during the night"¹⁰. The battery's usable capacity sets how much of that day-to-night shift a home can achieve.

How much usable storage different sized homes need

A typical home battery system is around 10 kWh⁴. That figure, and the sizing logic behind it, varies with the household.

• Average evening use: 1.5 to 3 kWh between sunset and bedtime for the average UK household, so a 5 kWh battery covers the evening with headroom¹.
• Recommended range: for most UK homes, 5 to 10 kWh covers evening use comfortably¹.
• Three-bedroom house: around 3,000 kWh of electricity a year, which averages out to a little over 8 kWh a day across the whole year¹¹.
• A real installation: one Energy Saving Trust case study household has an 8.2 kW home battery alongside solar panels and an electric vehicle, using battery power during the daytime, topped up by the panels¹².
• Modelling assumption: a Scottish housing retrofit study assumed a battery with 6 kWh of useful potential across all house types¹³.

The official MCS installation statistics group retrofit domestic batteries into bands of approximately smaller than 6 kWh, approximately larger than 11 kWh, and a grouping in between, which reflects how the UK market actually splits¹⁴. That matches the guidance: small households with modest evening use sit at or below 6 kWh, while larger homes, or homes with an electric vehicle or heat pump, sit at 8 kWh and above.

For a fuller worked approach to matching capacity to a household's consumption, see what size home battery you need.

Planning for UK conditions: overcast spells demand bigger batteries

Solar output in the UK is strongly seasonal, and that changes what a battery can do. The Consumer Protection Association's guidance on solar EV charging states the pattern directly: "In summer, a reasonably sized system can cover a significant portion of a typical household's EV charging needs. In winter output drops considerably and grid top-ups become more necessary"¹⁵. The same applies to household electricity generally.

The Insulation and Alternative Advice service makes the same point for heating-adjacent use: "if you're hoping to cover winter energy use, you may need an additional solution such as battery storage"¹⁶. In practice, a battery sized to cover the summer evening peak will be charging from the grid rather than the panels for much of December and January, because there is little or no surplus solar to store. Solar Energy UK's description of the mechanism still holds: "With a battery, any excess power produced during the day can be stored. This Electricity can then be used at another time"⁹. But in deep winter there is often no excess to store.

What this means for capacity is that winter performance is limited by generation, not by storage. A bigger battery does not create solar power on a cloudy December day; it only gives somewhere to put cheap off-peak electricity if the household is on a time-of-use tariff. Households aiming for high self-sufficiency in winter should expect to need grid top-ups, and can read more on solar battery self-sufficiency.

Common battery sizes and what they cost

The official parliamentary briefing on home energy storage reports that battery storage costs can be up to £10,000 depending on size, with a typical 5 kWh system around £4,600¹⁷. Energy Saving Trust gives the same headline figures: costs range from £1,500 to £10,000 depending on the type and size of battery, with a 5 kWh battery system around £4,600⁴.

More recent Scottish data narrows the picture for larger systems. An 8 to 10 kWh battery has a battery-only price of £2,800 to £4,500 and an installed price of £4,500 to £7,000, as at March 2026¹³. Independent UK guidance puts the starting point lower still: "Battery systems typically start from around £4,000"¹⁸.

A maker's buying guide states that its prices include labour and benefit from the 0 percent VAT that currently applies to residential battery installations until March 2027¹. Costs are covered in more depth on home battery cost.

How long batteries last, lose capacity and repay you

Independent guidance puts a typical battery's lifespan at about 10 to 12 years⁴. The Centre for Sustainable Energy gives a slightly wider band of 8 to 12 years, and notes that payback periods are "in the region of 8 to 12 years, which is similar to their reported lifespan"⁷. Home Energy Scotland says batteries tend to last between 10 and 15 years¹⁹, and the parliamentary research briefing on home retrofit measures states that "domestic battery storage has a shorter lifetime of around 10 to 15 years, when compared to solar panels"¹⁷.

On cycle life, a maker's guidance states that "modern home batteries are rated for 6,000 or more full charge cycles, which works out to roughly 15 to 20 years of daily" use¹. The same guidance notes that better battery chemistry costs a little more but usually pays off over a 10 to 15 year lifespan¹. Independent guidance is more conservative, putting the typical life at about ten years¹⁸.

The spread in these figures reflects what is being measured: a rating for full cycles is a laboratory figure, while a lifespan estimate accounts for real cycling patterns, temperature, and gradual capacity loss. Capacity does not switch off at the end of life; it declines, which is why warranties usually promise a retained capacity percentage after a set number of years or throughput, as covered on home battery warranties and cycle life and degradation. Because payback periods of 8 to 12 years sit close to the lifespan estimates, the financial case is examined on battery payback and savings.

Power cut protection and the stored energy it uses up

Not all batteries can deliver electricity during a power cut. The Centre for Sustainable Energy states this without qualification⁷, and Which? is equally direct: "If you're looking to protect yourself against power cuts with a home battery, not all systems are suitable"²⁰. Electricity North West adds that "in some cases these storage devices can provide back-up supplies for use in a power cut, but this is not always possible"²¹, while the Centre for Sustainable Energy notes that "the more expensive battery systems can also provide electricity during a power cut"²².

The connection to usable capacity runs in both directions. A battery held at its reserve floor has energy available for an outage, but only the reserve amount: a 20 percent reserve on a 10 kWh battery means roughly 2 kWh of backup, enough for lights, broadband and a fridge for a few hours, not for whole-house heating. Conversely, a household that runs its battery flat every evening has nothing left to fall back on. The reserve setting is therefore the mechanism that trades daily cycling against outage protection, and the equipment needed to disconnect the home from the grid during an outage is covered on battery backup and EPS and whether a battery works in a power cut.

The practical advice from consumer guidance is to ask the installer, before signing, whether the battery will work in an outage and for how long²⁰.

Approved installers, MCS accreditation and product standards

There is no single legal requirement that every domestic battery be MCS certified, but certification is woven through the rules that matter. The Warm Homes: Social Housing Fund scheme guidance requires that "for domestic batteries to be eligible for the WH:SHF scheme, the product must be installed by a MCS certified installer", working to the MCS battery installation standard, with UKCA or CE marking required, and installed in compliance with quality and installation standards including MCS (MIS 3012), PAS 63100 and PAS 2035²³.

The Centre for Sustainable Energy advises making sure your installer is MCS certified⁷, and Which? adds that installers should be signed up to the Renewable Energy Consumer Code, which now covers storage, and certified by MCS or alternatively by Flexi-Orb²⁰. Energy Saving Trust recommends getting at least three quotes from MCS certified installers⁴.

The official statistics on domestic battery installations are drawn from the MCS Installation Database, monitoring capacity and cost statistics for retrofit batteries across the United Kingdom²⁴. MCS's own consumer guidance describes the scheme's purpose as helping households "store excess energy to keep your home powered day and night"²⁵. The standards themselves are covered on battery installation standards.

VAT at zero today but set to increase

Since 1 February 2024, "electrical storage batteries installed in residential accommodation" have qualified for a temporary zero rate for VAT²⁶. HMRC's guidance for Great Britain states that you pay 0 percent VAT on both the qualifying products supplied by your installer and the cost of the installation work²⁷. A Scottish retrofit study confirms the position: "Since February 2024, there is a temporary 0% VAT applied to domestic battery storage systems"¹³.

The relief is time-limited. A maker's guide states that prices benefit from "the 0% VAT that currently applies to residential battery installations until March 2027"¹. After that, the domestic energy context matters: VAT is charged at the lower 5 percent level for domestic energy²⁸, and the MCS Foundation has campaigned against the loss of relief, warning of "an up to 20% jump in the cost of renewables like battery storage, solar PV and heat pumps" if the zero rate is not extended²⁹. The direction of travel on VAT for home energy is visible elsewhere too: ChargeUK reported in September 2026 that "home charging will now be zero VAT whilst public charging is levied at 20 per cent"³⁰.

The VAT position is covered in full on the VAT rate on home battery storage.

Grid independence depends on the capacity you can actually use

The core independence gain from usable capacity is substitution: electricity you store and use yourself is electricity you do not buy. Energy Saving Trust describes the combination plainly: "Combining solar panels with a home battery lets you store free, renewable electricity to power your heat pump. This makes you less reliant on grid electricity"³¹. In the case study household mentioned earlier, daytime power comes from the home battery, topped up by the solar panels, with excess energy exported to the grid¹².

The limits are just as real. A battery does not generate anything: it shifts energy in time, and in winter the solar surplus it depends on largely disappears, leaving grid charging as the alternative¹⁵. The system operator's own description of storage notes that for batteries "the electricity they can deliver is much more limited" than for pumped storage³², and the government's security of supply report describes batteries, both domestic and grid-scale, as storing electricity for up to eight hours³³. A domestic battery is an evening-shift device, not a generator.

What remains after installation is a set of dependencies worth naming: on the grid for winter charging and for power the battery cannot supply at once; on the manufacturer for the depth of discharge limits and the warranty that governs how much capacity is promised over time; and on the installer's configuration of the reserve. Usable capacity is the honest measure of all of this, because it is the figure that says what the battery will actually do for the household on an ordinary Tuesday evening. The wider picture is on home batteries and energy independence and in the site's main guide to home battery storage.