Retrofit solar battery: adding a battery to existing solar panels

How a retrofit connects

AC-coupled (most retrofits). The battery has its own inverter and connects to the home’s electrical supply next to the existing solar inverter. The panels and their inverter stay as they are. When the solar is producing more than the home uses, the battery inverter takes the surplus and stores it; when the home needs power, it gives it back. MoneySavingExpert notes the same trade-off most buyers face: an AC-coupled retrofit needs that extra inverter.

DC-coupled (a retrofit that replaces the inverter). The old solar inverter comes out and a hybrid inverter runs both the panels and the battery. It avoids converting the energy twice, but it means paying for a new inverter now. It is worth pricing if your existing inverter is old enough to need replacing soon anyway. The AC vs DC guide sets out the differences.

How many homes are retrofitting a battery

The Department for Energy Security and Net Zero now publishes the number of batteries registered with MCS in existing homes, which it calls retrofit installations, with new-build and commercial installations left out. In 2025/26 it counted 44,033 MCS-certified installations, 82% more than the 24,242 in 2024/25. Monthly installations rose from 405 in September 2023, the first month of the series, to a record 5,774 in March 2026, a 14-fold increase. The statistics are labelled as official statistics in development, they cover only MCS-registered batteries, and they do not say whether the home already had solar, but they are the best count of how many UK homes add a battery to a house that already exists.

They also say what those homes bought. In 2025/26 the average battery in the under-6 kWh band was 5.1 kWh, in the 6 to 11 kWh band 9.8 kWh and in the 11 kWh and over band 15.5 kWh. At each band’s mean price per kWh that is roughly £6,600, £8,500 and £10,400 for the average installation. That is roughly what the average household paid, including VAT where it applied and excluding anything fitted alongside the battery, such as solar panels.

What a retrofit costs

The official median across all MCS-certified installations was £890 per kWh for batteries of 6 to 11 kWh in 2025/26, which puts a 10 kWh battery at about £8,900 at the median. Standard supplier packages sit well below that; bespoke installations with backup, awkward cable runs or indoor fire-rated locations sit above it. Retrofit also costs more than fitting a battery with new panels: E.ON Next lists batteries from £1,900 with its solar and from £3,995 alone, and Octopus prices an extra Tesla Expansion Pack at £3,773 fitted with a new Powerwall and £5,909 added later. The cost guide has the full picture.

Will it pay back? Modelled by array and tariff

This site’s model puts a battery on a 2, 4 or 6 kWp array for a London home using 3,000 kWh a year, against the same home with the same panels and no battery on a flat tariff. The payback uses Octopus’s battery-only prices of £3,947 for 5 kWh and £5,336 for 10 kWh (Octopus’s 26 September rates):

Saving a year, simple payback in brackets. Octopus Go assumes the home qualifies for it, which it markets to electric car owners.

Three lessons follow. The tariff matters more than the panels. On a flat tariff a retrofit battery earns £106 to £211 a year whatever the array, because it can only store the surplus the home would have exported at 12p, and that takes 20 to 50 years to repay. A battery big enough to use the tariff matters next. On Flux a 10 kWh battery earns £370 to £414 a year, almost the same with a 2 kWp array as with 6 kWp, because most of the value comes from charging cheaply overnight and exporting in the 4pm to 7pm peak rather than from the panels; a 5 kWh battery is too small to cover the Flux peak and can earn less than on a flat tariff (£143 against £199 with a 6 kWp array). And the array’s size mainly decides how often the battery fills from solar in summer, not whether it pays. The model leaves out inverter replacement, backup value and any change in tariffs, so treat the paybacks as a way to compare options. Run your own array in the calculator.

Retrofitting a battery to Feed-in Tariff panels

Older panels are often on the Feed-in Tariff, and the worry is whether a battery puts the payments at risk. Ofgem’s guidance says storage can be added to an accredited FIT installation as long as the scheme’s metering rules are still met, and that the change must be reported to your FIT licensee with a diagram of the metering. An AC-coupled retrofit on deemed export is the safest combination: the generation meter still reads only the panels, so the generation payment and the deemed export payment are unaffected. Metered export is the fragile one, because a battery that can charge from the grid can stop the export meter isolating what the panels exported. The scenario-by-scenario rules are in will a battery affect my Feed-in Tariff. Octopus Flux adds a rule of its own: you can keep your FIT generation contract but must permanently opt out of deemed export.

The network question: G98 or G99

The capacity that counts is the total of every inverter that can export: the existing solar inverter plus the new battery inverter. The ENA’s G98 route covers up to 16 A per phase, which is 3.68 kW on a single-phase home, and lets the installer notify the network operator within 28 days of commissioning. A 3 kW solar inverter plus a 3 kW battery inverter adds up to 6 kW, which is over that threshold.

Above it, the installer makes a G99 application before fitting, and the network operator may allow the connection only with an export limit (a G100 scheme) that stops the home exporting more than an agreed amount. That is common and rarely a problem for a battery whose job is to use energy in the home, but it can limit how much stored energy you sell back on an export tariff. Ask who submits the application and how long it is taking in your area.

Does your array give a battery enough to store?

A battery added to solar earns in two ways: storing surplus that would otherwise be exported, and, on a time-of-use tariff, charging from the grid at cheap times. The first depends on the size of your array and the season; the second on the tariff.

• Large array, low daytime use: plenty of summer surplus. A battery earns from solar alone from roughly April to September.
• Small or older array: little surplus outside summer. The saving depends mostly on the tariff.
• Winter everywhere: PVGIS puts a 4 kWp south-facing array at about 145 kWh in December in London, a fraction of June’s output, so from November to February most of the battery’s value comes from cheap-rate charging.

Which tariff a retrofit can use

Octopus Flux is the tariff built for this: it needs a solar system and a home battery (they do not need to have been installed by Octopus), a smart meter with half-hourly readings, an export MPAN, both an import and an export tariff with Octopus, and the MCS document for the installation before the switch completes. If your panels have no MCS certificate, ask before you buy a battery on the strength of a tariff you cannot join. The current Octopus Flux rates for each region are on their own page, and the guide to the best tariff for a home battery covers the alternatives.

Export matters more when the panels are already there. Octopus’s export options pay a flat 12p, 16p in the evening peak on Prime or a half-hourly rate on Agile Outgoing, and Flux export pays about 27p to 31p in the peak.

Before you sign

• Check the installer is MCS certified for battery storage under MIS 3012, and which consumer code (RECC or HIES) protects your deposit.
• Ask for the usable capacity, the battery inverter’s power in kW and whether any backup is included.
• Make sure the battery location meets PAS 63100: outdoors or in a garage is usually easiest; bedrooms, hallways and lofts are ruled out.
• If your solar is on the old Feed-in Tariff, tell your FIT licensee and get the metering arrangement confirmed in writing, including whether your export is deemed or metered.
• Ask which tariff the installer’s payback assumes, and whether your home can join it.

Model your own array in the calculator, then compare two or three battery storage quotes for the same usable capacity.

If you want power in a cut, ask for home battery backup to be quoted as a separate line, with the power limit of the backup output.