What Size Solar Battery Do I Need UK 2026: A Sizing Guide

Why Battery Size Is About Your Evening Demand, Not Your Solar Array

Size your battery to the energy you use after the sun goes down, not the size of your solar array. This is the single most common mistake we see in quotes.

Installers love to pair batteries to panels. A 4kW array, they'll say, deserves a 10kWh battery. It sounds tidy. It's often wrong.

Here's why. Your solar panels produce electricity when the sun is up. Your house, in most cases, uses the lion's share of that electricity in real time: the fridge, the router, the hob at lunchtime, a load of washing. What a battery actually does is capture whatever is left over and hold it for the hours when the panels are producing nothing, roughly 6pm to 7am in a British winter.

So the question isn't "how big are my panels?" It's "how much electricity do I use between sunset and sunrise?"

And that number is almost always smaller than people think.

For a typical UK three-bed home with gas central heating, evening and overnight electricity use runs around 3 to 6kWh. If you have an all-electric home, a heat pump, or an EV charging overnight, that number climbs fast. The sizing question changes completely once a heat pump enters the picture, and we'll come back to that.

How to Estimate Your Usable Overnight Energy Needs

Look at your last 12 months of electricity bills and work out your daily average, then estimate what portion of that falls after 6pm. That's your target battery size, roughly.

Most UK households use between 2,700 and 4,100 kWh of electricity per year, according to Ofgem's typical domestic consumption values. Divide by 365 and you get a daily figure of roughly 7 to 11kWh.

But daily total isn't what you need. You need the evening and overnight slice.

As a rough rule, if you're out at work during the day, around 55 to 65% of your daily usage happens between 6pm and 7am. If you're home all day, it drops closer to 40%. Smart meters make this much easier. Log into your supplier's app (Octopus, EDF, OVO all show half-hourly data) and look at a typical weekday. The pattern tells you everything.

Here's the honest bit. Most guides skip the step of actually looking at your consumption data. They just quote a one-size-fits-all 10kWh battery. That's lazy and it's often expensive.

If your evening use is 4kWh, a 5kWh battery with 90% usable depth of discharge gives you 4.5kWh of real-world capacity. That's a near-perfect match. A 13.5kWh Tesla Powerwall in the same home would sit half-empty most nights, you'd have paid roughly £8,000 extra for storage you never touch.

The exception is if you plan to add electric heating, a heat pump, or an EV in the next few years. Then build headroom in now. A heat pump alone can push winter evening consumption to 15 to 20kWh.

Working Through a Sizing Calculation: A Hypothetical UK Home

For a three-bed semi with gas heating, two adults working from home two days a week, and no EV, around 5kWh of usable battery capacity is the sweet spot.

Let's walk through the numbers.

Annual electricity use: 3,400 kWh (slightly above Ofgem medium). Daily average: 9.3kWh. Evening and overnight share (6pm to 7am): roughly 55%, so 5.1kWh. Round down to 5kWh because weekends and holidays skew the average and you don't need to cover every edge case.

Usable capacity is not the same as nameplate capacity. A battery marketed as "5kWh" might only deliver 4.5kWh in practice because manufacturers protect the cells by preventing full discharge. We'll get to that in the next section.

For our three-bed semi, a battery with a nameplate of around 5.5 to 6kWh works. Something like a GivEnergy 5.2kWh unit or a Puredrive 5kWh. Both common on the UK market. Both come in around £3,500 to £5,000 installed depending on inverter setup.

And here's where tariffs matter more than most installers mention. If you're on Octopus Agile or Octopus Go, you can charge your battery overnight at around 7 to 10p per kWh and discharge during the 4pm to 7pm peak when grid electricity costs 30p plus. That changes the sizing logic. If you're playing the tariff arbitrage game, you want a battery that can hold a full day's evening use, closer to 8 or 10kWh. If you're only using it to store solar surplus, 5kWh is plenty.

How Power Limits and Usable Capacity Affect Your Choice

Two numbers matter beyond raw capacity: continuous power output (kW) and depth of discharge (DoD).

Continuous power is how fast the battery can deliver energy. A 5kWh battery rated at 2.5kW continuous means that even if it's full, it can only supply 2.5kW at any given moment. Fine for lights, fridge, telly, laptop. Not fine if you fire up a 3kW kettle while the oven's on at 2.4kW and someone's using the hair dryer.

In that scenario, the battery provides what it can, and the grid tops up the rest. You don't notice. But you don't get the full "off-grid feel" you might have expected.

If you want a battery that can run most of your home comfortably without pulling from the grid at peak times, look for 3.6kW continuous output or higher. Tesla Powerwall 3 does 11.5kW. Overkill for most people, but it's part of why it costs what it costs.

Depth of discharge is the other half of the picture. Lithium iron phosphate (LFP) batteries, which dominate the UK market now, typically allow 90 to 100% DoD. Older lithium-ion units sat around 80%. So a "10kWh" battery might give you 8kWh, 9kWh, or close to the full 10kWh depending on chemistry.

Always ask the installer for the usable capacity figure, not the nameplate. If they can't tell you, that's a flag.

Round-trip efficiency matters too, though less dramatically. Most modern batteries lose 10 to 15% of the energy you put in by the time you take it out. A 5kWh charge becomes 4.3 to 4.5kWh of delivered electricity. Build that into your expectations.

When Oversizing a Battery Actually Makes Sense

Oversize deliberately if you have a heat pump, an EV, a time-of-use tariff, or you're planning any of these in the next five years.

There are four scenarios where a bigger battery earns its keep.

First, time-of-use tariffs. Octopus Go, Intelligent Octopus, Agile, and similar tariffs from other suppliers let you charge from the grid at 7 to 15p overnight. If you can store 10 to 15kWh of cheap off-peak electricity and run your house on it from morning to evening, you avoid the 28 to 32p daytime rate entirely. The payback changes completely. You're no longer sizing to solar surplus, you're sizing to daily consumption. We've covered this logic in more detail in the Smart Export Guarantee guide, which looks at the export side of the same tariff game.

Second, heat pumps. A heat pump in winter can draw 15 to 25kWh per day, with the biggest demand in the evening when temperatures drop. A properly sized battery lets you run the heat pump off stored cheap electricity rather than peak-rate grid imports. If you're thinking about one, our heat pump efficiency guide covers how COP and SCOP affect those numbers.

Third, EV charging. If you charge an EV overnight on a cheap tariff, you don't need a battery for the car itself. But if you're adding a battery anyway for the house, sizing it to cover the house completely while the EV takes the overnight slot gives you clean separation and better optimisation.

Fourth, resilience. Some people genuinely value power cut protection. If that's you, a bigger battery with backup capability (not all batteries do this, check for "EPS" or "backup" in the spec) makes sense regardless of economics.

For everyone else, oversizing is wasted money. A 15kWh battery that half-fills every day will take 15 to 20 years to pay back at current prices. Most warranties run 10 years. The maths doesn't work.

Anyway, this is a separate discussion but it's worth flagging: battery prices are still falling about 8 to 12% per year. If you're borderline, waiting 18 months isn't a terrible plan. But that's a separate issue.

Can Any Open UK Grant Schemes Help Cover Battery Storage Costs?

No direct UK grant covers solar batteries in 2026. But a handful of indirect routes exist, and they're worth knowing.

Solar batteries themselves don't qualify for the Boiler Upgrade Scheme (that's heat pumps and biomass only, see our BUS guide) and they aren't a measure under ECO4 directly. The ECO4 scheme funds insulation, heating, and solar PV in some cases for low-income households, but battery storage specifically falls outside its scope.

Two routes are worth checking.

The first is the 0% VAT rule. Since April 2022, batteries installed alongside solar panels have qualified for 0% VAT, a saving of roughly £400 to £900 depending on system size. From February 2024, this was extended to retrofit batteries added to existing solar installations, and to standalone batteries. So a £5,000 battery install should attract zero VAT. If your installer tries to charge 20%, challenge it. The relief runs until 31 March 2027.

The second is the Warm Homes: Local Grant, administered by local authorities. The scheme focuses on insulation and low-carbon heating, but some councils have interpreted the guidance to include solar and battery storage as part of a wider retrofit package for eligible low-income households. Cornwall, Nottingham, and parts of Greater Manchester have been more generous than most. It's postcode-specific and criteria-specific, so the only way to know is to check your eligibility.

Honestly, don't bank on grant money for a battery. Price it on its own merits, factor in the 0% VAT, and if a grant route opens up through your council, treat it as a bonus.

One last thing. Some energy suppliers now offer "free" or heavily subsidised batteries in exchange for signing up to a specific tariff and giving them control over when the battery charges and discharges. Octopus and E.ON have both trialled versions of this. It can work out well, but read the terms. You're trading cash upfront for tariff lock-in, and the maths varies.

Open the eligibility checker. Two minutes. You'll know if any local scheme in your area is likely to help with the wider install.