Batteries and storage: does one suit your system?

A day's output, month by month

PVGIS 5.3 monthly output for Runcorn, 14% losses, read 3 November 2026. A typical household uses about 6.8 kWh a day across the year on Ofgem's 2,500 kWh figure, more in winter.

How often there is a surplus to store

Ofgem's medium household uses 2,500 kWh of electricity a year, about 6.8 kWh a day. The 4.4 kW south-facing system above averages more than that in 9 months of the year, and more than 10 kWh a day in 7. In the other months the panels on average make less than such a home uses in a day, so a battery would seldom fill from the panels alone. The surplus that makes storage worthwhile is a spring and summer surplus.

East-west pairs on a flat roof change the shape rather than the season. In June they average about 17.3 kWh a day against 17.0 from the south roof, but spread from early morning into the evening, so more of it lines up with the hours people are at home and less piles up at midday. For the same household, an east-west array usually leaves less for a battery to catch.

What a battery is worth

Each unit stored and used in the evening saves your unit rate (26.32p per kWh on average under Ofgem's cap for October to December 2026) instead of earning your export rate, less a little lost in the battery. That gap, times the units the battery actually shifts, is its yearly value. It is largest from April to September and small in midwinter.

A worked illustration, with every assumption stated: a battery that moves 4 kWh a day from afternoon into evening on 183 days of the year, from April to September, shifts about 732 kWh. At 26.32p saved instead of 5p earned, that is worth about £156 in a year, before the small loss inside the battery itself. Set that against the battery's price as a separate line on the quote; batteries are not in the government's per-kW averages on costs.

A battery and the connection

A battery can change how the system is connected. One that shares a hybrid inverter with the panels adds nothing to the total that could be exported. One with its own inverter adds that inverter's rating, so a 3.68 kW solar inverter with a 3 kW battery inverter beside it is over the single-phase limit for connecting first, and needs approval from SP Energy Networks before switch-on. The same applies when a battery is added to panels already on the roof. Grid connection sets out both routes.

Sizing and siting

• Look at your evening and overnight use; that is the most a battery can usefully cover each day.
• A battery much larger than the panels can fill will rarely be full.
• Ask whether the system can run some circuits during a power cut; many cannot without extra equipment.
• Agree where it goes: a garage, utility room or outside wall are common, with space for ventilation and access.

Winter, and charging from the grid

From November to January the 4.4 kW south roof averages between 4.0 and 5.6 kWh a day, less than the medium household uses, so a battery charged only by the panels sits largely idle in winter. Some households also charge a battery from the grid on a cheaper overnight rate and use that power at the evening peak, which keeps it working all year. Whether that pays depends on the tariff on offer, so treat it as a separate question from the solar sums; for help with the battery's cost, see grants and loans.

Questions for a battery quote

• How much of the stated capacity is usable, and at what rate can it charge and discharge?
• Will it share the solar inverter or have its own, and what does that mean for the connection?
• Can it charge from the grid on a cheaper overnight rate, and does your export tariff pay for what it sends out?
• Where will it be mounted, and what warranty covers it, in years and in cycles?

The first-year sums show how much the share of output used at home is worth; a battery is a way of raising that share.

Paying for it

Batteries in homes are zero-rated for VAT until 31 March 2027. The Warm Homes Loan Scheme has a separate £15,000 cap for an electrical storage battery. For eligible households, national guidance for the Warm Homes: Local Grant counts domestic batteries as a measure alongside solar panels.

Keep reading

• Is it worth it?
• Grid connection
• Costs
• Grants and loans

Where the figures come from

1. EU JRC PVGIS 5.3: solar output by location, tilt and direction
2. Ofgem: current price cap rates
3. Ofgem: typical household consumption figures (decision, May 2026)
4. Ofgem: Smart Export Guarantee for households
5. SP Energy Networks: getting connected
6. DESNZ statistics: small-scale solar PV installation costs, 2025/26
7. DESNZ: Warm Homes Loan Scheme rules v3 (September 2026)
8. DESNZ: Warm Homes: Local Grant policy guidance (June 2026)
9. GOV.UK: VAT relief on energy-saving materials (Notice 708/6)