A battery lets a home use its own solar in the evening instead of exporting it for a few pence. In this site's model a 4 kWp array on a home using 3,000 kWh a year covers about a quarter to a third of the output by itself; a 10 kWh battery roughly doubles that share. On Octopus Flux the same battery saves about £379 to £455 a year depending on region. Fitting both together is usually cheaper than adding the battery later.
What the battery adds to solar panels
Solar panels produce most of their energy in the middle of the day, when most homes use the least. Without a battery, the surplus goes to the grid and earns an export rate: 12p per kWh on Outgoing Octopus’s flat tariff, less on many others. With a battery, that surplus is stored and used in the evening instead of buying electricity at around 26p per kWh.
In this site’s model, a home using 3,000 kWh a year with a 4 kWp south-facing array uses about a quarter to a third of its solar output directly. Adding a 10 kWh battery roughly doubles that share. The rest is still exported, mostly on long summer days when the battery is already full.
Using the energy yourself is worth more than exporting it: Octopus’s flat export rate is 12p a unit, against about 26p to 37p for the electricity the battery lets you avoid buying on its standard and Go rates. The export-rate comparison shows what the other tariffs pay. And in a power cut the battery only helps if it was installed with backup; see how backup works in a power cut.
Why fitting both together costs less
- One inverter. A hybrid inverter runs the panels and the battery, instead of a solar inverter now and a second battery inverter later. Some batteries, such as Tesla Powerwall 3, build the solar inverter in.
- One visit and one connection. Scaffolding, electrical work, commissioning and the network notification are done once.
- Suppliers price it that way. E.ON Next lists batteries from £1,900 when fitted with its solar panels against from £3,995 on their own. Octopus prices an extra 13.5 kWh Tesla Expansion Pack at £3,773 fitted with a new Powerwall 3, and £5,909 if added later.
Published combined packages include Octopus at £11,115 for 12 panels and a 10 kWh battery, and Heatable’s median quote of £10,500 for solar with a battery.
The solar panels and battery cost guide sets these supplier prices beside DESNZ’s official averages and models how long the panels and the battery each take to repay; a battery on its own is priced in the solar battery cost guide.
Winter is the catch
The amount of surplus to store collapses in winter. PVGIS models a 4 kWp south-facing array in London producing about 490 kWh in June but about 145 kWh in December; in Glasgow the December figure is about 78 kWh. From November to February there is often no surplus at all after the home’s daytime use.
That is why the tariff matters. On a flat tariff a solar battery sits nearly idle in winter. On a time-of-use tariff it can charge from the grid in the cheap hours instead, so it earns all year. Octopus Flux, for example, is built for homes with solar and a battery: a cheap rate from 2am to 5am, a peak rate from 4pm to 7pm, and export at a higher price in the peak.
The Octopus Flux rates for every region are on their own page.
Sizing the battery to the panels and the house
Start from the house, not the roof. The battery earns only on energy it can move each day, so the useful size is roughly what the home uses between sunset and bedtime, plus any cheap-rate charging the tariff allows. For most homes using 2,700 to 4,000 kWh a year, that points to 5 to 10 kWh of usable capacity. Larger systems make sense with an electric car, a heat pump or a tariff that rewards exporting stored energy at the peak.
Also check the inverter. The battery’s charge and discharge power, set by the inverter, decides how fast it can fill in a cheap window and how much of the evening peak it can cover.
Paperwork to expect
- An MCS certificate for the solar (needed for Smart Export Guarantee payments) and for the battery installation under MIS 3012.
- A network notification: within 28 days of commissioning under G98 for a small system, or an application before installation under G99 if the combined inverter capacity is above 16 A per phase (3.68 kW single phase).
- An electrical installation certificate, with a fire-safety statement for the battery location.
Get the numbers for your home
Run your own usage, region and array size through the solar battery calculator, then ask installers for written quotes that list the usable kWh, the inverter and the backup included. Comparing more than one set of battery storage quotes is the quickest way to see whether a combined price is fair.
Questions people ask
Is it worth getting a battery with new solar panels?
Usually more worth it than adding one later, because the installation is shared. Whether it pays depends on the tariff: in this site's model a 10 kWh battery with 4 kWp of solar saves about £379 to £455 a year on Octopus Flux, but only £141 to £181 a year on a flat tariff, depending on region.
What size battery goes with a 4 kWp solar system?
For a typical home using 2,700 to 4,000 kWh a year, 5 to 10 kWh of usable storage captures most of the summer surplus. In winter a 4 kWp array rarely produces a surplus at all, so a larger battery only earns its keep if you also charge it from a cheap tariff.
How much does solar with a battery cost?
Published packages include Octopus at £11,115 for 12 panels and a 10 kWh battery, and Heatable's median quote of £10,500 for solar plus a battery (range £10,000 to £12,500), both 2026 figures. Installed home batteries and solar panels both carry 0% VAT until 31 March 2027.
Do I need a hybrid inverter?
For a new combined system a hybrid inverter, which runs both the panels and the battery, is the usual choice and avoids buying two inverters. Some batteries, such as Tesla Powerwall 3, have the solar inverter built in.
Can I get the Smart Export Guarantee with a battery?
Yes, for exported solar. Your solar installation and installer must be MCS certified. Suppliers are not obliged to pay SEG for battery exports that could have come from the grid, so if you want to sell stored energy at peak times, choose a tariff that allows it, such as Octopus Flux.