Eliot Crook, Founder · Updated 12 July 2026 · 9 min read
What Size Battery Do I Need?
For most UK homes, the right size is roughly one full day's overnight-shiftable consumption — commonly 5 kWh for a flat or small home, 10 kWh for a typical 3-bed family, and 13.5 kWh or more for larger homes, homes with an EV, or homes planning to add solar. Correct as of July 2026 — usage and tariff assumptions change.
The two questions that actually decide size
Everyone wants a single number, but battery sizing really comes down to two separate questions, and skipping either one is how people end up over- or under-buying. The first is: how much of your daily electricity use can realistically be shifted to cheap off-peak rates? The second is: how many hours of cheap off-peak electricity does your tariff actually give you to charge the thing up?
Get the first question wrong and you buy a battery bigger than your usage justifies — it never fully empties, and the extra capacity you paid for sits idle. Get the second wrong and you buy a battery that can never be filled in the off-peak window, so it caps out well below its rated size every single night. Sizing is really an exercise in matching capacity to both consumption and charging opportunity, not just picking the biggest box you can afford.
The "shiftable share" concept
Not every kWh you use is equally easy to move to off-peak hours. Baseload — fridge-freezers, routers, standby devices, that sort of always-on hum — tends to run 24/7 and typically makes up around 30-40% of a home's daily electricity use. A battery can cover this baseload through the day using energy stored overnight, which is one of the most reliable savings a battery delivers.
Some loads are effectively peak-locked: a kettle, oven or hob get used when they get used, and you're not going to time your dinner around a tariff window. These add to your daily total but don't really change how big a battery you need, because they happen whenever they happen.
Then there are the genuinely shiftable heavy loads: EV charging and heat pump run-time are both highly flexible, because you can choose to charge the car or let the heat pump's buffer tank run predominantly during the cheap window. These are usually the loads that push people towards a bigger battery, because they represent large, controllable chunks of daily energy rather than the odd kettle boil.
Calculating a starting point from your annual kWh
A simple, honest way to get a starting figure: take your annual electricity consumption in kWh and divide by 365 to get an average daily use. Then estimate what share of that is realistically shiftable — baseload plus any EV or heat pump charging you can move to off-peak hours — and treat that as roughly one day's target battery size.
It's worth adding around 20% headroom to that number for winter, when daily consumption is higher (more lighting, more heating-adjacent load, less solar offset if you have panels) and battery efficiency dips slightly in cold weather. This isn't about buying capacity you'll never use — it's about making sure the battery you buy actually covers a realistic winter evening, not just a mild September one.
This calculation gives you a sensible starting point, not a precise answer — real households have day-to-day variation, and tariff structure (covered next) can shift the right answer up or down from this baseline.
Matching size to your off-peak window length
A battery is only as useful as your ability to fill it. If your tariff gives you a 5-hour off-peak window — a fairly typical EV tariff shape — then the battery's continuous power rating (measured in kW, not kWh) has to be high enough to charge the full capacity in that time. A 10 kWh battery with a 3 kW charge rate needs over 3 hours just to fill from empty, which is fine within a 5-hour window; a 13.5 kWh battery on the same 3 kW charger would need well over 4 hours, leaving little margin.
This is a common oversight: households buy a large battery assuming it'll always fill overnight, then find it's only reaching 80% capacity because the charge rate and window length don't add up. Before sizing up, check the specific battery and inverter's continuous power rating against your tariff's off-peak hours — see our guide on /tariffs/economy-7 for how different off-peak windows compare.
When to size up
It makes sense to size up towards 13.5 kWh or beyond if you already have an EV and charge it substantially at home, if you have a heat pump with meaningful electric run-time, or if you're planning to add solar panels in future and want the battery to also soak up daytime generation rather than just overnight cheap-rate electricity. In all three cases, the extra capacity has a clear job to do rather than sitting half-empty.
When to size down
Conversely, it's sensible to size down towards 5 kWh (or even question whether a battery is worthwhile at all) if you live in a small flat or one-bed home with modest consumption, if most of your usage happens during the day rather than evening (working from home with daytime cooking/appliance use, for example), or if your tariff has a narrow spread between peak and off-peak rates. A bigger battery on a tariff with little price difference between day and night rates simply won't earn back its extra cost as quickly — see /guides/solar-battery-cost for how the maths plays out across sizes.
Decision table: usage band to recommended size
As a working shortcut, match your household type and annual consumption to the sizes below. These are starting recommendations based on typical shiftable-load patterns, not guarantees — always sanity-check against your own bills and off-peak window using the calculator.
Run it through the calculator for a precise answer
The table and workings above get you close, but the calculator turns this into a specific, tariff-aware recommendation. Head to /calculator, enter your annual kWh usage from a recent bill, tick the tariff you're on (or considering), and then adjust the battery size slider up and down. The tool will show you the estimated payback period for each size, so instead of guessing you can see directly whether stepping up from 5 kWh to 10 kWh actually shortens or lengthens your payback given your real usage and tariff spread.
This is particularly worth doing before committing to a larger, EV-influenced size, since the calculator accounts for your specific off-peak window rather than a generic assumption.
At a glance
| Household | Annual kWh | Recommended size | Reason |
|---|---|---|---|
| Flat / 1-bed | ~1,800 kWh/yr | 5 kWh | Low baseload, limited shiftable load — a larger battery rarely fully cycles |
| Small 2-bed | ~2,900 kWh/yr | 5 kWh | Modest daily consumption covers most of a 5 kWh capacity comfortably |
| 3-bed family | ~4,200 kWh/yr | 10 kWh | Higher baseload and evening cooking/appliance use fits a full day's shift into 10 kWh |
| 4+ bed home | ~6,000 kWh/yr | 13.5 kWh | Larger household load needs more capacity to cover a winter evening fully |
| Home + EV | ~8,500 kWh/yr | 13.5 kWh+ | EV charging adds a large, highly shiftable daily load on top of household baseload |
Frequently asked questions
What size battery do I need for a 3-bed house?
Around 10 kWh is the typical starting point for a 3-bed family home using roughly 4,200 kWh a year, assuming a reasonable off-peak window to charge it. Check your own annual usage on the /calculator to confirm.
Is a 10 kWh battery too big for a small home?
For a flat or small 2-bed home using under 3,000 kWh a year, a 10 kWh battery is often oversized — it won't fully cycle most nights, meaning you're paying for capacity you rarely use. A 5 kWh battery is usually a better fit for lower-usage households.
Does a bigger battery pay back faster?
Not automatically. Payback depends on how much of the extra capacity you actually use each night. If your shiftable load doesn't justify the larger size, a bigger battery can pay back slower per pound spent than a smaller, well-matched one — see /guides/solar-battery-cost.
What if I only have a 5-hour off-peak window?
Check the battery's continuous charge rate (kW) against the capacity (kWh). If it can't fully charge within 5 hours, you'll never use the full rated capacity overnight, so there's little benefit to buying a larger battery than the window allows you to fill.
Do I need to match my solar in kWp to my battery in kWh?
Not exactly — they solve different problems. Solar kWp is about generation capacity during the day; battery kWh is about storage for use later. A common approach is to size the battery to hold a decent share of your typical daily surplus, but the two don't need to be numerically matched.
Can I add more battery capacity later?
Many battery systems support modular expansion, but not all — and adding capacity later can cost more per kWh than buying it up front due to installation call-out costs. Check expandability with your installer if you're unsure whether to size up now or later.
What size battery do I need for a home with an EV?
Homes with an EV typically need 13.5 kWh or more, since EV charging is a large, highly shiftable daily load on top of normal household baseload. The exact size depends on how many miles you drive and how often you charge at home — run your numbers through the /calculator for a tailored figure.
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