Most UK homes with solar panels need somewhere between 8 and 15 kWh of usable battery storage, though the correct figure depends on your night time load, your battery’s depth of discharge, and how many days of backup you want. LiFePO4 chemistry usually sets the practical usable capacity, and installers size systems from that usable figure, not the rated one. The worked example and checklist below show exactly how to get from your own numbers to a final module size.
TL;DR:
- Most UK homes with solar panels need a battery capacity between 8 and 15 kWh, depending on nighttime energy use and backup requirements.
- Accurate sizing relies on analyzing twelve months of smart meter data or detailed evening appliance audits to determine realistic nightly energy consumption.
- Usable battery capacity is affected by depth of discharge and efficiency, with 48V systems typically being the practical choice for home installations.
- Planning for three or more days of autonomy is crucial in winter, as solar output drops significantly between November and February.
- Proper site surveys and professional installation prevent common mistakes like undersizing batteries or overestimating solar recharge potential.
Table of Contents
- Working out your daily energy use for home battery sizing
- Depth of discharge, efficiency, and usable capacity explained
- How many autonomy days should a UK home plan for?
- Converting Wh to Ah and choosing the right system voltage
- A worked example: sizing a battery for a typical UK home
- Does your solar array actually produce enough to fill the battery?
- What an installer checks before recommending your battery size
- The mistakes we see most often on-site
- Get your home battery sized properly with a site survey
- Sources
- FAQ
Working out your daily energy use for home battery sizing
Home battery sizing starts with one number: how many kilowatt-hours your house actually burns through, ideally split between day and night. Guessing here throws the whole calculation off, so it’s worth doing properly.
The most reliable method is pulling twelve months of smart meter data if you have it, because it captures winter heating spikes and summer lows rather than one unrepresentative week. Ofgem’s guidance on average gas and electricity use is a useful sanity check against your own figures, and government methodology for converting annual consumption into daily averages explains how official estimates handle that conversion.
If you don’t have twelve months of history yet, run a manual evening audit instead:
- List every appliance running between sunset and sunrise (fridge, lighting, TV, router, immersion heater, EV charger).
- Multiply each appliance’s wattage by the hours it runs, then convert to kWh by dividing by 1,000.
- Add standby loads: routers, smart speakers, and fridge compressors rarely switch off and often add 1 to 2 kWh a day between them.
- Add 10 to 15% for inverter losses, since power gets converted at least once between battery and appliance.
- Add a further 10 to 20% safety buffer to cover days you underestimated.
A three-bedroom house with a fridge freezer, router, lighting, and an evening of television and cooking typically lands somewhere around 8 to 10 kWh of night time draw once losses are included.
Depth of discharge, efficiency, and usable capacity explained
A battery’s rated capacity and its usable capacity are two different numbers, and this gap is where most sizing mistakes happen.
Depth of discharge (DoD) is the percentage of a battery’s total capacity you can safely draw down before recharging.
Round‑trip efficiency (RTE) describes how much energy you get back out compared with what went in, after conversion losses in the battery and inverter. Sizing guides and calculators consistently flag DoD and round‑trip efficiency as the two biggest levers on how much installed capacity you actually need.
The formula installers use is straightforward:
- Installed capacity = Usable kWh needed ÷ (DoD × RTE)
That gap between “what you need” and “what you buy” only grows with lead‑acid chemistry, where 50% DoD pushes the same 9 kWh requirement to 18 kWh installed.
How many autonomy days should a UK home plan for?
Autonomy days measure how long your battery could run the house with zero solar input and no grid import, and this is where UK weather starts to matter.
Installer guidance on solar battery bank sizing generally splits recommendations into three tiers:
- Grid‑tied homes using the battery mainly to shift energy use and cut import costs: 0.5 to 1 day of autonomy is usually enough.
- Comfort or partial backup for freezers, lighting, and internet during outages: 1 to 2 days.
- Whole‑home backup, including heating and cooking: 3 or more days, which usually needs a larger array and battery bank than most retrofit budgets allow.
Winter is the wrinkle UK homeowners often miss. Solar generation drops sharply between November and February, so a battery sized purely on summer surplus will underperform exactly when you need backup most. Our guide to how solar performs through a UK winter breaks down the seasonal shortfall in more detail.
Pro Tip: If backup power during outages is genuinely mission‑critical, size for your worst winter day’s consumption, not your annual average. Oversizing slightly for the lean months costs less than a generator when the power actually goes.
Converting Wh to Ah and choosing the right system voltage
Battery capacity gets quoted in kWh for marketing, but the wiring and hardware are specified in amp hours (Ah) at a given voltage. The conversion is simple:
- Ah = Wh ÷ V. For an 11 kWh (11,000 Wh) battery at 48V, that’s 11,000 ÷ 48, or roughly 229 Ah.
- Pick your system voltage. Most UK domestic installs now use 48V, because it delivers more power at lower current, meaning thinner, cheaper cabling and fewer losses than 12V or 24V setups.
- Round up to the nearest available module size. Battery manufacturers sell in fixed increments, typically 5 kWh or 10 kWh blocks, so your 11.1 kWh calculation becomes a 15 kWh purchase in practice, not a custom‑built 11.1 kWh unit.
- Never mix battery ages or brands within the same bank. Uneven cells drag down the whole pack’s performance and can trip safety cutoffs.
- Check your inverter’s continuous and surge ratings against the battery’s output before finalising anything, since an undersized inverter bottlenecks a perfectly good battery.
24V or 12V systems still turn up in smaller cabins, boats, or off‑grid outbuildings, but for a family home, 48V is the practical default.
A worked example: sizing a battery for a typical UK home
Here’s a complete calculation you can copy with your own figures.
Usable requirement: 9 kWh × 1 day = 9 kWh
Installed capacity before margin: 9 ÷ (0.80 × 0.90) = 12.5 kWh
With 15% ageing margin: 12.5 × 1.15 = 14.4 kWh
Rounded to nearest module: 15 kWh
At 48V, that’s roughly 15,000 ÷ 48 = 312 Ah
That 15 kWh figure sits comfortably in the range most three to four bedroom UK homes land on. Two other common scenarios show how far the number swings either way:
An EV charging overnight, or a heat pump replacing gas heating, can add several kilowatt hours to that night load figure. If either is on your horizon within the next few years, size the battery bank (or at least the inverter and wiring) to accommodate the larger future draw rather than repeating this whole calculation from scratch later.
Does your solar array actually produce enough to fill the battery?
Buying a bigger battery than your panels can recharge each day is a common and expensive mistake. Before finalising a size, check what your array can realistically deliver.
- A rough rule of thumb used across the industry is 0.8 to 1.5 kWh of usable storage per kWp of installed solar, though this varies with roof orientation, shading, and your local UK sunlight hours.
- A 4 kWp array producing a genuine summer surplus might comfortably fill a 5 kWh battery daily, but struggle to fill a 15 kWh unit outside the brightest months.
- If your chosen battery size exceeds what your panels can realistically recharge, you have three options: add more panels, accept partial recharge through winter, or expand the battery bank in stages as your array grows.
- Shading from chimneys, trees, or neighbouring rooflines can quietly cut output well below the theoretical kWp figure, so it’s worth checking our note on how shading affects solar output before committing to a large bank.
What an installer checks before recommending your battery size
A proper site visit goes well beyond the arithmetic above. Here’s what a qualified installer should be checking on‑site before quoting a final size:
- Meter history and load profile, including any EV charging or hot water immersion patterns that spike consumption.
- Roof orientation and shading, since a south‑facing, unshaded roof recharges a battery far more reliably than an east/west split with tree cover.
- Existing or planned EV and heat pump loads, which change the sizing maths significantly.
- Earthing arrangements and EPS (Emergency Power Supply) requirements, if backup during a grid outage matters to you.
- BMS (Battery Management System) settings, confirming the DoD and safety cutoffs match what was quoted.
- Inverter continuous and peak power ratings, checked against the battery’s actual output capability, not just its headline kWh figure.
- Cabling, isolation switches, and warranty documentation, all of which should be itemised on any written quote.
Whoever you choose to install your system, ask for MCS certification, written performance guarantees, and a clear service plan before signing anything. The installer should carry relevant industry accreditations and hold high customer satisfaction ratings, built on thorough site‑survey discipline as outlined above.
The mistakes we see most often on-site
Undersizing is the error that costs homeowners twice: once on the battery, then again on an upgrade a year later. Skipping DoD in the initial maths, or ignoring how much winter production drops, causes most of the regret calls we get.
Priority order matters. Get the safety margin right first, pick a chemistry that holds its value over a decade of cycling, and treat serviceability as seriously as sticker price. Anything beyond simple evening‑only top‑upsizing deserves a proper site survey, not a spreadsheet guess.
— Simon
Get your home battery sized properly with a site survey
Working through the maths above gives you a realistic ballpark, but roof shading, EV plans, and your actual meter data can shift the final number by several kilowatt hours either way. A qualified installer carries out the site survey, runs the sizing calculation against your real consumption data, and handles supply and installation as one job rather than passing you between separate suppliers and fitters.
A typical visit covers your roof orientation, existing consumer unit, and any EV or heat pump plans, followed by a written quote showing the exact battery size recommended and why. Once installed, the Solar & Battery Service & Maintenance Plan covers ongoing performance checks for £175 a year, so BMS settings and inverter health get monitored rather than left until something fails. If you’re ready to move from calculation to quote, get in touch through the solar and battery installation page to book a survey.
Sources
For further reading beyond this guide, Ofgem’s average use data, the Energy Saving Trust’s battery storage advice, and independent battery sizing calculators are worth checking against your own figures.
- Average gas and electricity use explained – Ofgem
- Consumption level review methodology (March 2025) – UK government
- Battery capacity calculator — Roboculator
FAQ
How big is a 10kWh home battery?
A 10 kWh home battery typically fits in a space roughly the size of a large suitcase or small wall cabinet, wall‑mounted in a garage or utility room.
How long will a 10kWh battery power a house?
A 10 kWh battery running only essentials (fridge, lighting, router, TV) can often last a full day or more, while a house running a heat pump or electric cooking might drain it in 6 to 10 hours. The exact figure depends entirely on your household’s actual night time load, which is why calculating your own consumption matters more than any generic estimate.
What are the battery sizes in the UK?
UK domestic battery products are typically sold in fixed increments, commonly around 5 kWh, 10 kWh, and 15 kWh, with larger homes sometimes stacking multiple modules to 20 kWh or beyond. Energy Saving Trust guidance frames typical household needs within this range, though the right size still depends on your night load and autonomy goals.
Is a 5kWh battery enough?
A 5 kWh battery suits smaller households or flats using it mainly to shift evening electricity use onto stored solar rather than for outage backup. For a family home with higher night time consumption or any backup ambitions beyond a few essentials, 5 kWh usually falls short, and the worked example above shows how quickly that figure grows once DoD and autonomy are factored in.
