Solar battery sizing UK 2026: kWh capacity for a typical household

Solar battery sizing in the UK is mostly an arithmetic problem dressed up as a sales conversation. The right kWh for a household is set by three values: the daily evening import load (the kWh used after the panels stop generating), the average summer surplus the array produces, and the per-kWh cost of the battery against the per-kWh value of the avoided imports. Most installer quotes pitch 10 kWh because that is the SKU with the best gross margin, not because the household needs it. This guide walks through the sizing arithmetic with UK-specific load profiles.

Step one: get the evening load right, not the annual total

Annual kWh is the wrong unit. A battery is sized to cover the load between the array switching off (around 5-7pm in summer, around 3-4pm in winter) and the array starting again the next morning. That window is the "evening load" plus the early-morning load.

Ofgem's typical domestic consumption values, last updated in April 2025, set the electricity-only medium-home figure at 2,700 kWh/year. That works out to roughly 7.4 kWh/day on average. Around 55-65% of that load falls outside daylight hours, which means the evening-plus-morning import window is typically 4-5 kWh in a non-EV, non-heat-pump household.

A larger home with an EV charging on a smart tariff at 2am does not "need" battery capacity to cover the EV; the EV is on its own off-peak window already. A heat pump running through winter night is a different story, because the heat pump draws when the battery would otherwise be sitting empty post-sunset.

Step two: match the household profile to a kWh band

The figures in the right column are "usable" capacity. A 5 kWh nameplate battery typically delivers around 4.5 kWh usable because most lithium-iron-phosphate (LFP) units cap discharge at around 90-95% depth of discharge to preserve cycle life. Read the spec sheet for "usable energy" not nameplate.

Step three: check the summer surplus actually fills it

A battery is useless if the array cannot fill it. A 4 kWp South West array on a sunny day in June generates around 25-30 kWh, of which roughly 5-7 kWh is used by the home in real time. The remaining 18-25 kWh is potentially available to charge a battery, which is comfortably enough to fill a 9.5 kWh unit. The catch is winter. The same 4 kWp array in mid-December in Manchester might generate 3-5 kWh on a sunny day and 0.5-1.5 kWh on an overcast day. There is no surplus to charge anything beyond a token level. A 13.5 kWh battery in a Manchester install sits at 10-20% state of charge through most of January and February.

That is fine if the buyer is paying for the summer dividend. It is not fine if the buyer was sold the battery on the basis of winter self-sufficiency, which it cannot deliver.

Step four: cost per usable kWh, not headline price

2026 retail benchmarks for a fitted residential battery sit roughly in these bands.

The cost-per-usable-kWh band is U-shaped: tiny batteries are expensive per kWh, mid-range batteries are cheapest, very large batteries climb back up slightly because of the stacked-unit or premium-brand effect. Verify the current quote against installer-specific scope before signing.

Step five: model the annual return per usable kWh

The cleanest sizing test is: how much extra annual benefit does each marginal kWh of battery deliver?

For a 4 kWp Midlands array, the first 3 kWh of battery captures most of the daily summer surplus most of the year (around 240 cycles a year at full depth). At 27p/kWh saved and an 8p/kWh export rate avoided, each cycled kWh is worth around 19p net. Annual benefit: roughly £137 from the first 3 kWh.

The next 3 kWh (between 3 and 6 kWh) cycles less often (perhaps 180 cycles a year at full depth) because not every day generates enough surplus to fill it. Annual benefit: roughly £103.

The next 3 kWh (between 6 and 9 kWh) cycles less still (perhaps 110 cycles a year). Annual benefit: roughly £63.

Above 9 kWh the marginal cycle count drops below 70 a year. Annual benefit per added kWh sits below £40. On a £600 marginal cost per kWh that pushes payback beyond 15 years for the top end of the battery, which is longer than the battery's useful life.

EV smart tariffs change the maths

A household on Octopus Go, Intelligent Octopus, EDF Go Electric, or E.ON Next Drive imports overnight at 7-9p/kWh rather than the cap rate of around 27p. The economic value of a kWh saved from a battery is therefore not 27p; it is the difference between the daytime peak rate the battery is displacing and the marginal cost.

Here is where it breaks for the "bigger is better" battery pitch. If the household can already shift large discretionary loads (EV charging, dishwasher, washing machine) to the off-peak window at 7-9p, the marginal value of an extra 5 kWh of battery is much lower than for a non-smart-tariff household.

In practice the sweet spot for a smart-tariff household is often 5-9 kWh, not 13.5 kWh, because the battery is competing with cheap off-peak import rather than expensive peak import.

Heat pumps push the answer up

A heat pump draws electricity through every cold evening, often at 1.5-3 kW continuous for several hours. The post-sunset load in a 4-bedroom heat-pump home in February is regularly 8-12 kWh between 5pm and 10pm alone.

The Climate Change Committee's 2025 Sixth Carbon Budget progress report flagged household electrification as the single biggest residential demand driver into 2030, with heat pumps adding 30-50% to typical winter peak imports. For a heat-pump household, 9.5 kWh usable is a sensible floor and 13.5+ kWh is reasonable.

Warranties, cycles and what to actually compare

Battery warranties typically guarantee 60-70% remaining capacity after either 10 years or a fixed cycle count, whichever comes first. The cycle count is the variable that matters and the one most quotes do not surface clearly.

Typical 2026 figures: GivEnergy units around 6,000 to 10,000 cycles depending on model. Tesla Powerwall 3 unlimited cycles with a 10-year date cap. Sunsynk ECCO 6,000 cycles. Pylontech 6,000 cycles at 80% depth of discharge.

A battery cycled once a day reaches 3,650 cycles in 10 years. A battery cycled 1.5 times a day (which a well-sized 5 kWh unit in a Midlands home approaches in summer) reaches 5,500 cycles. The catch is that a small battery cycled hard hits its warranty cycle count before its date cap; a large battery cycled lightly hits its date cap first.

The DNO and the 3.68 kW export limit

Most UK home batteries that can export to grid (rather than just charge from solar and discharge to home) are subject to the G98 connection limit of 3.68 kW per phase. Above that, a G99 application to the Distribution Network Operator is required and may take 2-12 weeks to process.

For a Bristol install with Western Power Distribution as the DNO, G98 notification is typically same-day or next-day. For a rural Scotland install with SSEN, G99 timelines can stretch. On the ground a buyer planning a battery larger than 5 kWh with grid-export capability should ask the installer to confirm the DNO permit position before signing.

Frequently asked questions

What size battery does a typical 3-4 person UK home need?

For a 3-4 person household with gas heating and no EV, a 5 kWh usable battery (commonly sold as a 5.2 kWh nameplate unit) covers most evening load and lifts self-consumption from around 35% to around 65%. Heat pump or multiple-EV households should aim higher.

Is 13.5 kWh overkill?

Usually yes, unless the household has a heat pump, multiple EVs charging on different schedules, or runs frequent evening peak-shaving on an Octopus Agile or similar dynamic tariff. For a standard gas-heated home, 13.5 kWh sits idle most of the year.

Can a battery be added later?

Yes, and the HMRC zero-rate VAT on standalone retrofit batteries since 1 February 2024 removed the price penalty that previously applied. A retrofit AC-coupled battery can be added to most existing solar installs, though wall space, distribution-board capacity and DNO permit must be confirmed.

Does a battery work during a power cut?

Only if the inverter has explicit Emergency Power Supply (EPS) or whole-home backup functionality and the installer wires the backup circuits at install. Most standard installs do not include backup by default and the battery shuts down for safety when the grid goes down.

What is the most important spec to compare?

Usable kWh (not nameplate), guaranteed remaining capacity at end of warranty (60-70% is normal), warranty cycle count, and round-trip efficiency (90-95% for current LFP units). Cost per usable kWh is the cleanest cross-brand metric.

How long does a battery last in the UK climate?

LFP chemistry units installed indoors or in temperature-controlled garages typically reach 10-15 years before falling below useful capacity. Outdoor installs in unheated outbuildings degrade faster because deep cold (below 0C) reduces charge acceptance.

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