Home Battery Capacity: What Size Do You Need?
5kWh, 10kWh or something larger? The right size depends on your consumption, your solar surplus and what you want the battery to do. This guide walks you through it step by step.
- Independent advice
- 5-30+ kWh covered
- Free sizing tool
- Size the battery to the electricity you use in the evening and at night, not to your total annual use. That is the hard ceiling on what storage can ever save you.
- Most households without an EV or heat pump land between 5kWh and 10kWh.
- With an EV or a heat pump, 10kWh to 15kWh is the usual range.
- Above around 6,000 kWh a year, 15kWh to 25kWh.
- Capacity is only half the question. Power decides what the battery can actually run, and on a plug socket that is limited to 800 W however many kilowatt-hours you buy.
- A single modular system tends to top out somewhere around 20kWh, depending on the platform. Beyond that you are running units in parallel, which changes what your network operator needs from you.
Mike's take
Everyone shops on kilowatt-hours and almost nobody asks about kilowatts. Ask what the system delivers on a dedicated circuit, not what it holds. A big battery behind a small output is a slow battery, and no amount of extra capacity fixes that.
What does battery storage capacity mean?
Capacity is how much energy a battery can store, in kilowatt-hours (kWh). It says nothing about how fast that energy can go in or out. That is power, in kilowatts (kW), and it is set by the inverter and by how the battery is wired into your home.
Think of a swimming pool. The volume of the pool is capacity in kWh. The pump that fills and empties it is power in kW. A big pool with a small pump still empties slowly.
Capacity only pays for itself if it actually gets used. A battery that is full by 2pm and empty by 9pm still leaves you buying expensive evening electricity for half the night. Getting the size right is less about buying as much storage as you can afford, and more about matching it to how your household uses electricity after the sun goes down.
Why power matters as much as capacity
This is the part most buying guides skip, and it changes which battery is right for you more than any capacity figure does.
Modular home batteries are typically limited to around 800 W of discharge when they run from an ordinary earthed socket. The limit exists so the battery cannot overload a circuit it shares with other appliances, and it applies whether the stack holds 5kWh or twenty. Connected instead to a dedicated circuit by a qualified electrician, the same hardware commonly delivers two to three kilowatts, depending on the model.
| Evening load | Typical draw | Covered at 800 W? |
|---|---|---|
| Fridge, lights, router, TV | 300-500 W | Yes |
| Washing machine mid-cycle | 500-2,000 W | Partly |
| Kettle | 2,000-3,000 W | No |
| Electric oven | 2,000-3,000 W | No |
| Induction hob, one ring | 1,500-2,000 W | No |
Read that table again before you choose a capacity. A 20kWh battery held behind an 800 W output cannot carry a British evening peak, because cooking alone exceeds it. The dedicated circuit is not an upgrade for large systems; it is what lets a battery of any size do the job you bought it for.
Nominal and usable capacity are not the same
Manufacturers advertise nominal capacity, the physical size of the battery. What you actually get out of it each day is usable capacity, because the battery management system holds back a reserve to protect the cells. Most lithium iron phosphate batteries release 90 to 95% of nominal, so a 10kWh battery gives you roughly 9 kWh in daily use.
This matters when you compare products. A system advertised at 5kWh and one advertised at 5.3kWh may deliver almost the same usable energy, and a headline figure on its own tells you very little. Look for the usable figure, not the nominal one.
Work out your own number in two steps
The configurator below does this properly, using your actual consumption and export data. But if you want a rough figure by hand first, two calculations get you most of the way there. Our colleague Colin walks through exactly this method on camera, if you would rather watch it than read it:
Or follow the same two steps written out below.
Find your average daily use
Take your total annual consumption in kWh from your last electricity bill and divide it by 365. A 4,000 kWh year works out to roughly 11 kWh a day. That is your baseline, not your final answer: if you are home during the day and use solar directly, your battery needs will be lower. If you are out at work and your use is concentrated in the evening, they will be higher.
Convert to the nominal size you need to buy
The formula is required usable energy divided by the usable capacity percentage equals required nominal capacity. Wanting 7 kWh of usable storage from a battery with a 90% usable limit means 7 divided by 0.90, so roughly 7.8 kWh nominal. In that case, a standard 8 kWh system is a sensible starting point.
Choosing the wrong size is one of the most expensive mistakes you can make.
Recommended capacity per household
Four things set the real answer: your total annual consumption, how much of that falls in the evening and at night, whether an EV or heat pump is already in the house or on the way, and how much solar you have to recharge from during the day. As a starting point:
| Annual consumption | Situation | Recommended capacity |
|---|---|---|
| Under 2,000 kWh | One or two people, small flat | 5 kWh |
| 2,000-4,000 kWh | Family, no EV or heat pump | 5 to 10 kWh |
| 4,000-6,000 kWh | Family with an EV or heat pump | 10 to 15 kWh |
| 6,000-9,000 kWh | Large family with EV and heat pump | 20 to 25 kWh |
| Over 9,000 kWh | Large property, electric heating throughout | 30 kWh or more |
These are starting points, not answers. Two households with identical annual bills can need very different batteries if one is empty all day and the other is not. Run the configurator below for a figure based on your own data.
Too small or too large
Getting the size wrong rarely means the system stops working. It just quietly costs you money, in one of two directions.
Too small
- Fills within a few sunny hours and exports the rest anyway
- Empty before midnight, so you still buy peak electricity
- Fixable later if the system is modular
Too large
- Rarely fully charged, especially in winter
- You paid for capacity that never cycles
- Extra modules add standby consumption
Of the two, too large is the harder mistake to undo. A modular system can usually grow, but nobody will refund you for capacity that sits idle eleven months of the year.
How far a modular system scales
A modular battery is a base unit with its own electronics plus expansion modules that add storage. You start small and add modules later, which is useful when your budget is limited, your future consumption is uncertain, or an EV charger or heat pump is planned.
There are two ceilings worth knowing before you commit to a platform, and both vary from brand to brand, so treat these as the shape of the market rather than fixed numbers. A single unit tends to top out somewhere around 20kWh, once the base unit carries its full complement of expansion modules. Beyond that you add a second and then a third complete unit, running in parallel, which takes the largest residential systems to roughly 60kWh. Capacity keeps climbing; what changes at that point is not the battery but the paperwork, which the next section covers.
Check before you buy
- Is expansion officially supported, and up to how many kWh per unit?
- Can units run in parallel, and how many?
- Must added modules match the age, capacity or firmware of the first?
- Will the same modules still be sold in three years, and at what price?
Done well, modular expansion means you are not paying today for capacity you will not need until that EV or heat pump actually arrives. Done badly, it means buying a discontinued module at a premium, or discovering that expansion was never supported on your model in the first place.
Socket, dedicated circuit and what your network operator needs
In Great Britain the rules that matter here are not about how much energy a battery stores but about how much power it can push back into your wiring. There are three steps, and which one you land on follows directly from the capacity you choose.
Socket connection
The battery runs from an ordinary earthed socket with discharge capped at around 800 W. Simple, but it cannot carry an evening peak, and current wiring rules expect anything that feeds power into your home to sit on a circuit of its own. Treat this as a starting configuration rather than the finished article.
Dedicated circuit, one unit
A qualified electrician runs a circuit of its own from the consumer unit, which lifts output to two or three kilowatts on most platforms. A single unit stays under the 3.68 kW single-phase threshold, so the installer can connect it and notify your network operator afterwards. This is the normal route for anything a single unit can hold, which is usually around 20kWh.
Dedicated circuit, units in parallel
Two or three units together reach roughly 6 kW to 9 kW, which is above that threshold, so your network operator has to approve the installation before it goes in rather than hear about it afterwards. Allow time for it. This applies to systems beyond a single unit, broadly 25kWh and up.
Wiring rules are also moving. The current edition of the wiring regulations added dedicated provisions for stationary battery storage, and the government has commissioned a safety study into plug-in batteries whose findings are expected to shape whether, and how, socket-connected storage is permitted in future. If you are buying now, assume a dedicated circuit and a qualified electrician.
What home battery storage costs
With a modular system the hardware is the bulk of the bill and the connection is a comparatively small, fixed addition. That is a very different shape from a conventional system built around a hybrid inverter, where installation is a large share of the quote.
| Capacity | Hardware | Dedicated circuit | Grid connection |
|---|---|---|---|
| 5 kWh | £1,100 – £1,400 | £250 – £500 | Connect and notify |
| 10 kWh | £2,000 – £2,500 | £250 – £500 | Connect and notify |
| 15 kWh | £2,900 – £3,600 | £250 – £500 | Connect and notify |
| 20 kWh | £3,800 – £4,700 | £250 – £500 | Connect and notify |
| 25 kWh | £4,700 – £5,800 | £250 – £500 | Prior approval |
| 30 kWh | £5,500 – £6,900 | £250 – £500 | Prior approval |
Three things the table does not show. Modules bought after the first cost less per kilowatt-hour, because the base unit’s electronics are already paid for, so a growing stack drifts towards the bottom of its range. A conventional installed system with a hybrid inverter is a different product at a different price, commonly several thousand pounds more for the same storage, and it buys you higher power and fuller backup rather than more kilowatt-hours. And payback depends far more on your electricity contract than on the purchase price: a household on a time-of-use tariff that charges cheaply overnight and discharges through the evening peak recovers the investment considerably faster than one on a flat tariff with a generous export rate.
Sizing a battery without solar panels
A home battery can be worth having with no solar panels at all. On a time-of-use tariff, electricity prices change through the day. The battery charges when prices are low, usually overnight, and supplies the house when prices are high. That changes how you size it: there is no solar surplus to capture, so the only question is how many kilowatt-hours you can shift from cheap hours to expensive ones.
What decides the benefit
- The gap between cheap and expensive hours
- Whether your output is high enough to cover the peak you are avoiding
- Round-trip efficiency of the battery
One cycle a day
Price arbitrage normally means one charge and one discharge per day. Your saving is the daily price spread multiplied by your usable capacity, so the sum is easy to run for any size.
Check before you switch
Not every battery supports automatic price-based charging, and the charge rate decides whether you can fill the whole stack inside a cheap window.
Charge rate deserves a moment here. A system that charges at 3 kW fills about 9 kWh in a three-hour overnight window. If your stack is larger than that, you will not refill it in one cheap period, which quietly caps what the extra capacity can earn. Size the battery to the window, not just to the tariff.
Frequently asked questions
What is the difference between nominal and usable capacity?
Nominal is what the label says; usable is what the battery management system releases. Divide the storage you need by the usable percentage: 9 kWh needed at 90% usable means roughly 10kWh nominal.
Why is a plug-in battery limited to 800 W?
Because an ordinary socket shares a circuit with other appliances, and a battery feeding power onto that circuit alongside them could overload it. The cap protects the wiring. Moving the battery to a dedicated circuit removes the shared load, which is why output typically rises to two or three kilowatts there.
How much battery storage do I need per kWp of solar?
A rough starting point is 1 to 1.5 kWh of storage per kWp installed. It is only a starting point. Your consumption pattern matters more than the ratio, because storage you cannot discharge in the evening earns nothing.
Does winter change the answer?
Yes. Sizing on peak summer production gives an oversized system that spends half the year partly empty. Use a full twelve months of data, which is exactly what our hourly model does.
Why not just buy the biggest battery I can afford?
Beyond the point where it covers your evening and night use, extra capacity rarely cycles. You end up paying for kilowatt-hours that sit idle most of the year, when that budget is often better spent on more solar, a heat pump, or simply saved.
Is a 5kWh or a 10kWh battery better?
Neither is better in itself. A 10kWh system only beats a 5kWh one if you have enough solar surplus to fill the extra space and enough evening demand to empty it again. Without both, you are paying twice for capacity that cycles half as often.
What is the largest home battery I can install?
It depends on the platform, but as a guide a single modular unit reaches somewhere around 20kWh, and three units in parallel take the largest residential systems to roughly 60kWh. Above a single unit the combined output crosses the threshold at which your network operator must approve the installation in advance.
Do I need an electrician?
For anything on a dedicated circuit, yes, and that is the configuration most households should be planning for. A new circuit is notifiable work, so it has to be done by a registered electrician or signed off by Building Control, and the certificate matters later when you sell the house.
Get the size for your own home
The calculator uses your consumption, your solar export and your local tariffs to recommend a capacity, and shows the payback that comes with it.