Solar battery sizing comes down to one number: how many kilowatt-hours the home uses when the panels are not generating, between late afternoon and the next morning. Get that right and the customer self-consumes most of their solar instead of exporting it cheaply. Get it wrong and you either oversell hardware or leave savings on the table. With the average UK home now using around 2,700kWh of electricity a year, roughly 7.4kWh a day (Energy UK, 2025), most households land on a battery between 5kWh and 14kWh.
This guide sets out the sizing method installers actually use on survey: work out daily consumption, adjust for usable capacity and depth of discharge, then match the battery to the evening and overnight load rather than the array size. It also covers self-consumption gains, oversizing trade-offs, and how an EV or heat pump changes the answer.
Key Takeaways
- Size to evening and overnight load: match usable capacity to the electricity used from roughly 4pm to 8am, when solar is not covering demand (EcoFlow, 2025).
- Start from daily kWh: annual consumption divided by 365 gives daily use; the average UK home is about 7.4kWh a day (Energy UK, 2025).
- Usable is not nominal: modern LFP batteries offer 90% to 95% depth of discharge, so a 10kWh nominal battery gives about 9kWh to 9.5kWh usable (GreenMatch, 2025).
- Batteries lift self-consumption sharply: solar self-consumption typically rises from 30% to 40% up to 70% to 80% once storage is added (SolarTherm, 2025).
- Slight oversizing beats undersizing: a 10% to 20% margin covers future demand and keeps cycles shallow; a battery that is too small caps the savings permanently.
How do you size a solar battery?
Start with daily consumption. Divide the annual figure from the customer's bills by 365, or read the smart meter's daily average. A home on 2,700kWh a year averages about 7.4kWh a day (Energy UK, 2025). Then estimate how much of that falls outside daylight generation, because that overnight and evening slice is what the battery has to cover.
A common working method is to take the daily kWh the battery needs to cover, add a 10% to 20% margin, then divide by the usable share of the battery to get the nominal capacity to specify. For an LFP battery at 90% depth of discharge that means dividing by 0.9 (GreenMatch, 2025). The result is a nominal figure to quote, not the usable one, so always confirm which number a manufacturer's spec sheet is showing before you compare products.
What size battery does a typical UK home need?
Most UK homes settle between 5kWh and 14kWh of usable storage, with a typical three-bedroom household on around 9kWh to 10kWh a day landing on a 9kWh to 12kWh battery for roughly 80% self-sufficiency (Sunsave, 2025). The list below maps rough daily consumption to a starting battery size before adjusting for solar array output and habits.
- Low use, ~5kWh/day (1 to 2 bed flat): a 5kWh usable battery covers most evening and overnight demand.
- Medium use, ~7.4kWh/day (average UK home): a 7kWh to 8kWh usable battery is a sensible starting point (Energy UK, 2025).
- Higher use, ~10kWh/day (3 to 4 bed): a 9kWh to 12kWh battery targets around 80% self-sufficiency (Sunsave, 2025).
- With an EV: add roughly 10kWh to 15kWh on top of the base figure, so a 3-bed with an EV often needs 16kWh to 20kWh total (Sunsave, 2025).
- Large array, high daytime export: size up so more surplus is captured rather than exported at low rates.
Usable capacity vs nominal: why depth of discharge matters
Nominal capacity is the headline number on the box; usable capacity is what the customer actually gets. Modern lithium iron phosphate batteries run at 90% to 95% depth of discharge, so a 10kWh nominal unit delivers roughly 9kWh to 9.5kWh usable (GreenMatch, 2025). Quoting the nominal figure as if it were usable is the most common sizing error, and it leaves the customer short every evening.
Always size against usable capacity and state it clearly on the quote. Compare products on the same basis too, because a 9.5kWh usable battery and a 10kWh nominal battery with 90% depth of discharge are effectively the same storage. Round-trip efficiency and inverter losses trim a little more, so build a small margin in rather than sizing to the exact daily figure. It is also worth noting that a battery run to full depth every day ages faster than one cycled gently, so a slightly larger unit that rarely empties can outlast a smaller one worked hard, which matters over a warranty measured in thousands of cycles.
How much does a battery improve self-consumption?
A well-sized battery is the single biggest lever on self-consumption. Without storage, a typical home self-consumes only 30% to 40% of what its panels generate, exporting the rest at low Smart Export Guarantee rates. Add storage and that figure typically climbs to 70% to 80% (SolarTherm, 2025), which is why battery storage now features in over 30% of new UK solar installations (Atlantic Renewables, 2025). The saving is real: shifting that much generation from a low export rate to displaced grid import is usually worth several hundred pounds a year on current tariffs.
The size of that gain depends on matching capacity to the overnight load. A battery too small to hold the evening surplus caps the benefit, while one far larger than the array can fill on a dull day sits half-empty and takes longer to pay back. Model the customer's own generation and consumption profile rather than reaching for a default 10kWh unit. A retrofit solar battery on an existing array is sized the same way, working from measured consumption.
Sizing for self-consumption, backup or tariff arbitrage
The right size shifts with the customer's goal. For maximising self-consumption, match usable capacity to the evening and overnight load. For time-of-use tariff arbitrage, where the battery charges cheaply overnight and discharges at peak, size to the peak-period demand plus a margin, and a slightly larger battery often pays back through the tariff spread. For backup, size to the essential circuits the customer wants to keep running and for how long.
These goals can pull in different directions, so agree the priority before quoting. A customer chasing tariff arbitrage with an AC coupled battery storage setup may want more capacity than self-consumption alone would justify, while a customer purely offsetting daytime export needs less. Whichever the driver, a correctly sized hybrid inverter has to be able to charge and discharge the battery fast enough to use that capacity within the window.
Should you oversize or undersize a battery?
Where there is doubt, lean slightly large. A 10% to 20% margin over the calculated figure covers future demand such as an EV or heat pump, keeps each cycle shallower and gentler on the cells, and avoids the battery hitting empty most evenings (EcoFlow, 2025). Undersizing permanently caps the savings, and adding a second battery later is rarely as clean as fitting the right size once.
There is a ceiling, though. A battery much larger than the array can realistically refill will spend days part-charged, which lengthens payback without adding usable savings. The sweet spot is a battery the solar can fill on an average day and the household can empty most evenings. That balance, not the biggest number the customer can afford, is what a good survey delivers.
How does an EV or heat pump change battery sizing?
Both add significant overnight and shoulder-period load, so both push the battery larger. An EV typically adds 10kWh to 15kWh of demand on charging nights, so a three-bedroom home with an EV often needs 16kWh to 20kWh of total storage for good self-sufficiency (Sunsave, 2025). Much EV charging, though, is better done directly from a cheap overnight tariff than round-tripped through the battery.
A heat pump raises winter electricity use sharply, exactly when solar generation is lowest, so a battery sized on summer figures will fall short in the heating season. Size on the customer's real annual profile and be honest that in mid-winter the battery mainly shifts cheap overnight grid electricity rather than storing solar. Setting that expectation avoids a disappointed customer in January.
Sizing a battery on survey: the installer checklist
On survey, pull twelve months of consumption from bills or the smart meter, not a single month, because seasonal swing is large. Note the array size and orientation, the customer's daily routine, whether an EV or heat pump is present or planned, and the priority between self-consumption, backup and tariff savings. Those inputs, not a rule of thumb, decide the number.
Then size against usable capacity, add a modest margin, and check the inverter can move that energy within the charging window. A common mistake is fitting plenty of battery behind an inverter that cannot charge or discharge fast enough to use it inside the cheap-rate window, which strands capacity the customer paid for. Tools that pull consumption, generation and battery capacity into one model, such as Reonic's installer platform, make it easier to show the customer the self-consumption a given size will actually deliver rather than quoting a default and hoping. A sized quote the customer understands closes better than a guessed one.
Frequently asked questions
How do I calculate what size solar battery I need?
Divide annual consumption by 365 for daily use, estimate the share used outside daylight, add a 10% to 20% margin, then divide by the usable share (about 0.9 for LFP) to get the nominal capacity to specify (GreenMatch, 2025). Size against evening and overnight load, not the array size.
What size battery does an average UK home need?
With the average home using around 2,700kWh a year, about 7.4kWh a day, a 7kWh to 8kWh usable battery is a sensible starting point (Energy UK, 2025). A larger three or four-bedroom home on 10kWh a day usually lands on 9kWh to 12kWh for roughly 80% self-sufficiency.
What is the difference between usable and nominal capacity?
Nominal is the headline figure; usable is what you can actually draw, set by depth of discharge. Modern LFP batteries run at 90% to 95%, so a 10kWh nominal battery gives about 9kWh to 9.5kWh usable (GreenMatch, 2025). Always size and compare on usable capacity.
How much will a battery increase self-consumption?
Typically from 30% to 40% without storage up to 70% to 80% with a well-sized battery (SolarTherm, 2025). The exact gain depends on matching capacity to the overnight load, so model the customer's own generation and consumption rather than assuming a fixed uplift.
Is it better to oversize or undersize a solar battery?
Slightly oversizing, by 10% to 20%, is usually better. It covers future demand, keeps cycles shallow and avoids emptying the battery each evening (EcoFlow, 2025). Undersizing permanently caps savings. Avoid going far larger than the array can refill, as that just lengthens payback.
