What size solar battery do I need? A UK guide to 6kWh, 12kWh and the numbers behind the choice

What size solar battery do I need: the short answer, then the method

A solar battery is sized to the electricity you use when your panels are not producing: every evening, and all of every winter day. It is not sized to the number of panels on your roof. Two houses with identical ten panel systems can need different batteries if one runs the dishwasher at 7pm and the other at 1pm while the sun is still out.

The method is three numbers:

1. How many kWh you use in a day, from your bill or smart meter.
2. What share of that falls after dark, when the battery, not the grid or the panels, is doing the work.
3. What your panels actually produce on the days that matter, which in winter is a fraction of a summer day.

Put those three together and you get a target capacity in kWh. Pro-Fit publishes two sizes, 6kWh and 12kWh, and a free survey confirms which one, or whether solar alone makes sense first, fits your house. Other configurations are specified at survey.

The three numbers that decide it

Your daily electricity use in kWh

Your energy bill states usage in kWh, usually monthly or annual. Divide the annual figure by 365 for a rough daily average, though households vary day to day. A smart meter or in-home display gives the same number more precisely, often by half hour.

Ofgem publishes Typical Domestic Consumption Values (TDCV) as a reference point for low, medium and high electricity use. Its current figures, effective from 1 July 2026, are low 1,600kWh, medium 2,500kWh and high 3,800kWh per year for a standard single-rate meter (Ofgem, "Review of typical domestic consumption values", decision published 27 May 2026, ofgem.gov.uk, read 7 October 2026). Treat it as a benchmark only: TDCV bands are built from a national spread of households, not yours, so your own bill or meter reading is what sizes your battery.

The table below turns those bands into a rough, illustrative starting point. It is not a substitute for the method above.

Indicative only. Household size is a rough proxy for Ofgem's consumption bands, not an official correlation, and the evening-and-overnight share is an illustrative estimate, not a published industry figure (see below for why no such figure exists). A household with an evening-heavy routine, an EV or a heat pump will sit well above its row here. Your own bill or smart meter, worked through the method above, is what actually sizes your battery; Pro-Fit's two sizes, 6kWh and 12kWh, both carry headroom over these illustrative minimums.

How much of that you use after dark

This is the hardest number to get a clean published figure for. Energy Saving Trust, discussing battery storage, says there's "no exact formula for working out what you need, but your installer will be able to advise you based on your needs and circumstances" (Energy Saving Trust, "Battery storage", energysavingtrust.org.uk/advice/battery-storage/, last updated 6 October 2026, read 7 October 2026). There is no single, dated UK figure for "the evening share of the average home."

So treat it as an estimate from your own smart meter, not a published constant. If your display or app shows half-hourly use, add up everything from roughly 4pm to bedtime, plus overnight draw from a fridge, freezer or device left running, and compare it with your day's total. That evening-and-night figure, not your whole day's use, is what the battery has to supply once the sun goes down.

Rule of thumb: divide your evening-and-overnight kWh by the usable share of a battery's nominal capacity (stated on your quote) to get the nominal size to ask for.

What your panels produce and when

Pro-Fit fits four array sizes, 6 panels (2.79kWp) to 12 panels (5.58kWp). What an array produces on a given day depends heavily on season and weather, not just its kWp rating. On a clear summer day, even a 6 panel array comfortably covers daytime use and fills a 6kWh battery well before the afternoon. On a grey winter day, the same array can produce a fraction of that, sometimes not enough to fill a 6kWh battery, because UK winter daylight is short and the sun sits low. A worked example has to show both days separately: an annual average hides the problem a battery is bought to solve.

A worked example in kWh

Take a household using 10kWh on an average day, of which it estimates from its smart meter that 4kWh falls in the evening and overnight window.

On a strong summer day: a 6 panel (2.79kWp) array produces more than the house uses all day. A 6kWh battery fills from the surplus well before evening and comfortably covers the 4kWh evening and overnight use, with capacity to spare. A 12kWh battery would also fill, but carries unused capacity through most of the summer.

On a weak winter day: the same array produces far less, and daytime use takes a larger share of it before anything reaches the battery. A 6kWh battery may only partially fill from solar alone, covering what it has stored, with the rest drawn from the grid or an off-peak overnight charge (see below). A 12kWh battery gives more headroom to bank what solar there is plus an overnight top-up, with capacity left for the next evening if poor generation runs several days.

The figure that moves this decision is the evening-and-night kWh total, not the panel count. A household with a bigger evening load, say 6 to 8kWh after dark, pushes towards 12kWh even on a modest array, since the battery is covering more of its own use rather than topping up from solar surplus.

Battery terms that trip people up

Nominal versus usable capacity and depth of discharge

A battery's nameplate or "nominal" capacity is not all of it you get to use. Manufacturers reserve a margin to protect the cells, expressed as a maximum depth of discharge (DoD), so usable kWh is typically below the nominal figure, though the gap varies by model. Sigenergy's SigenStor product page states "100%" against "depth of discharge, maximum energy utilisation", with a footnote clarifying this refers to usable energy capacity (Sigenergy, SigenStor product page, sigenergy.com/products/sigenstor, read 7 October 2026), meaning its nominal and usable figures are presented as closely aligned; other designs differ. Ask for both figures on your quote.

Physical size and where it goes

A typical home battery is roughly 100cm x 60cm x 25cm (Energy Saving Trust, "Battery storage", energysavingtrust.org.uk/advice/battery-storage/, last updated 6 October 2026, read 7 October 2026). It is usually wall-mounted or floor-standing in a garage or utility room, or fixed to an external wall where the property and local rules allow; exact size varies by model and capacity, and where yours goes is confirmed at survey.

Power (kW) versus capacity (kWh)

Capacity (kWh) is how much energy a battery stores. Power (kW) is how fast it can deliver or accept it. A battery with plenty of capacity but a low power rating can still struggle if a kettle, oven and shower run at once, with the shortfall drawn from the grid automatically. Your installer sizes the inverter and battery power rating to your peak demand, not just your daily total, confirmed on your quote.

Round-trip efficiency

No battery returns 100% of the electricity put into it; some is lost as heat charging and discharging. This loss, round-trip efficiency, varies by chemistry, inverter and configuration, so the figure for your hardware is stated on your quote rather than one number fitting every model.

Cycles and warranties

A "cycle" is one full charge and discharge. Manufacturers warranty batteries against a stated cycle count, years, or both. Sunsynk rates its LiFePO4 batteries at "6,000+ charge cycles at 80% depth of discharge" with a 10-year warranty as standard (sunsynk.org, read 7 October 2026), and Powervault's P5 carries a 10-year warranty as standard (powervault.co.uk, read 7 October 2026). Fox ESS publishes a modular residential range, its EP and EQ series, spanning roughly 5kWh to over 40kWh in a single stack (fox-ess.com, read 7 October 2026). Exact cycle counts and warranties differ by model and are confirmed on your quotation.

What size battery for solar panels: 6kWh or 12kWh

6kWh: smaller households, daytime-heavy use, a first battery

A 6kWh battery suits a smaller household, or one whose evening and overnight use is modest, perhaps 3 to 5kWh. It also suits anyone adding a first battery to a new or existing solar system, to see how storage behaves on their own roof and bills before committing to a larger unit.

12kWh: larger households, overnight running, night-tariff charging, EV or heat pump on the way

A 12kWh battery suits a larger household, one that routinely uses 6kWh or more after dark, or runs appliances overnight. It also suits anyone planning to charge from an off-peak or night tariff (see below), since the larger capacity gives more room to bank cheap overnight electricity alongside solar surplus. If an electric vehicle or heat pump is likely within a few years, a 12kWh battery gives more headroom from day one, though your survey confirms what your plans require.

When a battery does not make sense yet

A battery does not make sense yet for a household that uses most of its electricity during the day, when the sun already covers it, with little evening or overnight draw to store against. Solar panels alone are worth it there, and Pro-Fit's solar-only tier is built exactly for this case, with a hybrid inverter that is battery-ready from day one so storage can be added later.

Charging from the grid: off-peak tariffs and why they change the sizing maths

Many suppliers offer time-of-use tariffs, with a cheaper unit rate during set off-peak hours, typically overnight, and a higher one otherwise. The supplier sets the windows and the rates, and a battery that shifts its charging into the cheap window pays less for the same electricity. We describe the mechanism here, not any supplier's rates, since prices change and are a matter between you and your supplier.

For sizing, a household on a time-of-use tariff can treat the cheap overnight window as a second source of charge alongside solar. That favours a larger battery, since there is more worth banking overnight on a short winter day when solar alone would leave it half empty. Pro-Fit sets up time-of-use charging for your tariff on handover day.

Export: what happens to surplus once the battery is full

Once your battery and household demand are satisfied, surplus solar electricity is exported to the grid. Under the Smart Export Guarantee (SEG), participating suppliers must pay small-scale generators for electricity they export, setting their own rates and contract terms, with SEG rates always required to be above zero (Ofgem, "Smart Export Guarantee", read 7 October 2026). Eligibility for a particular supplier's SEG tariff depends on that supplier's own requirements; Pro-Fit's installation support includes help registering for export payments, but rates sit between you and your supplier.

A bigger battery means less is exported on an average day, since more generation is used on site. A smaller battery exports sooner. Neither is "better" in isolation; it depends on whether you value using your own electricity in the evening more than the export payment, which comes back to your evening use.

Winter: the season a battery earns its place

Summer is when a battery looks easiest to fill and empty, since generation comfortably exceeds most households' use. Winter is the harder test: shorter days, lower sun angles and more cloud cut generation sharply, exactly when heating and lighting push use up. A battery sized only against a summer day can look generous in July and undersized in January. For sizing, treat your winter evening use as the figure to be comfortable with.

Backup in a power cut: what a standard battery does and does not do

A standard grid-tied installation, including Pro-Fit's 6kWh and 12kWh systems, does not keep your house running in a power cut. Hybrid inverters disconnect from the grid automatically during an outage for safety, and a standard installation lacks the separate wiring needed to isolate and power circuits independently.

Whole-home backup is a different, separately specified option: a battery with backup capability, paired with backup board wiring for chosen circuits, so they keep running through a power cut. This is specified and priced in writing after a free survey, since the circuits covered vary by property.

Adding a battery later

If you start with solar only, or already have solar without storage, a battery can be added later. Pro-Fit's solar-only tier uses a hybrid inverter that is battery-ready from day one. An existing system from another installer can usually also be retrofitted, through a compatible hybrid inverter or an AC-coupled battery alongside the existing inverter; which route suits your system is confirmed at survey.

Standalone batteries, and batteries added to an existing solar system, qualify for the same 0% VAT relief as a new combined installation. HMRC confirms that batteries retrofitted to an existing microgeneration system, and standalone batteries storing electricity from the grid or from microgeneration, qualify for the zero rate in residential accommodation, with the relief running from 1 February 2024 and reverting to the reduced 5% rate after 31 March 2027 (HMRC, VAT Notice 708/6, read 7 October 2026).

What a Pro-Fit battery system costs

Every figure below is a guide price, fixed in writing after a free survey, for a standard pitched roof.

Every system includes DMEGC 465W all-black panels, a hybrid inverter that is battery-ready from day one, a battery set up for off-peak charging where selected, scaffolding put up and taken down, bird protection around the panels, a monitoring app set up before we leave, grid connection paperwork and a handover pack, and help registering for export payments.

Whole-home backup has no published price: it is specified and priced in writing after your free survey.

0% VAT on qualifying installations until 31 March 2027, then 5%.