Last reviewed: 5 September 2026. This page sizes a battery from your
own consumption data, which your supplier must give you free. Installed price ranges are published by
UK installers, not a price survey of our own. Electricity figures use the Ofgem price cap for 1 October
to 31 December 2026.
The battery you need is the one that just about empties itself most evenings. Not the one that fits
your roof, not the one the installer has in the van, and not the one that would carry you through a
power cut for three days. Capacity that never gets cycled earns nothing, costs several hundred pounds
per kilowatt-hour and ages anyway.
Oversizing is the most expensive mistake in the domestic battery market, and it is a mistake that
almost every part of the sales process pushes you towards. This page pushes the other way.
The only question that matters
How much electricity do you use between the end of your cheap overnight window and the start of the
next one — in practice, roughly 7am to midnight — and specifically how much do you use in the expensive
evening block?
That figure is your discharge requirement. Everything else is detail. And it is almost always
smaller than people guess, because a mental model of “we use loads in the evening” does not survive
contact with half-hourly data.
Get the data, do not estimate it
- Ask your supplier for your half-hourly smart meter consumption data. It is yours, and it is
free. - Pick a fortnight in a cold month and a fortnight in a mild one.
- Sum the half-hours from 4pm to 10pm on each weekday. That is your peak block.
- Take the typical figure, not the worst one. Sizing for your highest ever day means buying
capacity that sits idle for 360 days a year.
The sizing table
The table works from annual consumption to a starting capacity, then shows what that capacity is
worth at a 15p spread and what it costs. Treat the middle column as a starting hypothesis to test
against your own half-hourly data, not as an answer.
| Annual electricity use | Typical daily use | Rough evening block | Starting usable capacity | Annual saving at 15p spread | Typical installed cost |
|---|---|---|---|---|---|
| 1,800 kWh — flat, one or two people | 5 kWh | 1.5 – 2 kWh | 3 – 5 kWh | £142 – £236 | £2,500 – £4,500 |
| 2,700 kWh — Ofgem typical household | 7.4 kWh | 2 – 3 kWh | 5 kWh | £236 | £2,500 – £4,500 |
| 4,000 kWh — family, gas heating | 11 kWh | 3 – 4.5 kWh | 5 – 8 kWh | £236 – £378 | £3,000 – £6,000 |
| 6,000 kWh — family plus an EV charged at home | 16 kWh | 4 – 6 kWh, plus the car overnight | 8 – 10 kWh | £378 – £473 | £4,000 – £7,500 |
| 9,000 kWh+ — heat pump, or all-electric | 25 kWh | 7 – 10 kWh | 10 – 15 kWh | £473 – £709 | £4,000 – £8,500+ |
Two structural observations from that table. Cost per kWh falls as capacity rises, because the
inverter, the electrician’s day and the paperwork do not scale with the box — which is the honest
argument in favour of going a size up. And annual saving rises linearly with capacity only for as long
as you actually empty it, which is the argument against. The two meet at the size that runs flat most
evenings.
The EV row deserves a warning. A car charged overnight on a cheap rate does not need the battery at
all; it is already buying at the off-peak price. Do not let anyone size a battery to “cover the EV” —
that is capacity bought to duplicate something your tariff already does for free.
Why oversizing costs real money
Say your evening block is 4 kWh and you are offered 10 kWh usable at £6,000 rather than 5 kWh at
£3,500. The extra £2,500 buys capacity of which you cycle perhaps a fifth. At a 15p spread the 5 kWh
unit, emptied nightly, saves about £236 a year; the 10 kWh unit, half-emptied nightly, saves about the
same — because the saving comes from the energy moved, not from the energy sitting there.
| Usable capacity | kWh actually cycled per night | Annual saving | Indicative installed price | Simple payback |
|---|---|---|---|---|
| 3 kWh | 3 — battery empties, some peak import remains | £142 | £2,500 | 17.6 years |
| 5 kWh | 4 — matched | £189 | £3,500 | 18.5 years |
| 10 kWh | 4 — half the capacity idle | £189 | £6,000 | 31.7 years |
| 15 kWh | 4 — two-thirds idle | £189 | £8,500 | 45 years |
The bottom two rows are money set on fire. Note also that at a 15p spread and this modest an evening
block, none of these paybacks is good — which is the more important lesson. If your evening use is
small, the honest advice is that a battery may not be for you at any size. Run your own numbers through
the standalone battery arithmetic before choosing
between sizes.
Two numbers that matter as much as capacity
Usable capacity, not nominal
A “10 kWh” battery may hold 10 kWh nominal and permit 9 kWh of usable discharge, or less, depending
on the depth-of-discharge limit the manufacturer sets to protect cell life. Every figure on this page
is usable. Insist that every quote you compare states usable capacity in writing, or you are comparing
nothing.
Continuous power output in kW
Capacity is the size of the tank; continuous power is the width of the pipe. A 10 kWh battery rated
at 3 kW continuous cannot carry an oven, a kettle and a tumble dryer simultaneously — the shortfall is
imported at peak rate, exactly when you were trying not to.
Look at the highest half-hourly figure in your evening data and double it to get an instantaneous
kW estimate. If your peaks routinely exceed the battery’s continuous rating, a bigger tank will not
help; a wider pipe will.
Sizing for backup is a different exercise
If your reason for buying is power cuts rather than money, ignore everything above and size from the
loads you want to keep alive and for how long — a fridge, freezer, router, some lights and a phone
charger is on the order of 2 to 3 kWh a day. Two points people get wrong: backup functionality is not
included as standard and needs additional switching, which must be asked for and priced separately; and
a battery configured for backup usually reserves a slice of capacity permanently, which reduces what is
available for arbitrage and therefore lengthens the payback in the tables above.
Room to grow, and whether to buy it
- Modular systems let you add capacity later. That flexibility usually carries a
price premium, and it is worth paying only if a specific change is coming — a heat pump, an EV, or
someone moving in. - Do not size for a heat pump you might install in five years. Battery prices per
kWh have been falling; capacity bought early is capacity bought expensive and aged for five years
before it is needed. - Do size for a tariff change you are confident about, because the tariff is the
engine of the whole case.
The warranty check that constrains size
Warranties typically run ten years and frequently carry a total-throughput cap in MWh alongside the
retained-capacity percentage. Do the sum: a 10 kWh battery fully cycled 350 times a year moves 3.5 MWh
a year, or 35 MWh over ten years. If the warranty caps throughput at, say, 24 MWh, then daily cycling
exhausts the warranty around year seven, not year ten — and the payback tables assume you keep cycling.
A battery you intend to work hard needs a warranty sized for that work. See battery warranties.
Frequently asked questions
Should I buy bigger because VAT goes up in 2027?
No. The rate goes from 0% to the reduced 5% on 1 April 2027, not to 20%. On a
£5,000 job that is £250 — not a reason to buy capacity you will not cycle. See buying a battery after the VAT change.
Does having solar change the size I need?
With a substantial rooftop array, yes — the battery then has a second job, soaking up midday surplus,
and a larger unit can be justified. With an 800 W plug-in kit, no: the surplus involved is far too small
to change the sizing. See can I add a battery to
plug-in solar.
What if my consumption changes?
A battery slightly too small still earns its full value every night; a battery too large never earns
the difference back. When genuinely undecided between two sizes, the smaller one is the lower-risk
error.
Sources.
Ofgem, price cap unit rates and standing charges ·
HMRC, VAT Notice 708/6, energy-saving materials.
Installed price ranges are published UK installer guidance, labelled as such; all sizing and payback
figures are our own arithmetic from the stated assumptions. This is general information, not financial
advice — see editorial methodology.