Adding a battery to solar panels you already own

Yes, you can, and you probably should

An array installed before batteries were affordable is doing half a job. It generates well in the middle of the day when the house is empty, exports the surplus for whatever the export tariff pays, then buys that same energy back at the capped rate of 26.11p per kWh when everybody gets home. A battery closes that loop without touching a single panel.

The retrofit route is well trodden and the economics are usually better than they look, because the expensive parts of a solar installation are already paid for. Scaffold, roof work, the array itself and the original grid connection are all behind you.

Almost every retrofit is AC coupled

An AC coupled battery brings its own inverter and charger and connects on the AC side of your system. Your existing solar inverter stays exactly where it is, your panels are untouched, and the battery can be sited wherever is practical rather than next to the array.

The alternative is replacing your solar inverter with a hybrid unit and going DC coupled, which is more efficient but only makes financial sense if the existing inverter is old and due replacement anyway. Our guide to AC and DC coupling works through the trade in detail.

The DNO rule that catches retrofits

This is the one thing people are consistently not told before they sign. Your Distribution Network Operator assesses your connection on inverter rating, not on how many kWh your battery stores, and crucially it assesses cumulatively across the whole site.

So if you have a 3.5 kW solar inverter and you add a 3.5 kW battery inverter, the site is now rated at 7 kW. That is well beyond the G98 threshold of 16A per phase, which is 3.68 kW on a single phase supply, and it means the job needs a G99 application approved in writing before installation. Ofgem's connection guaranteed standards give the network operator 45 working days to issue a quotation for a low voltage generation connection, with an £80 failure payment if it misses that, and installers report type tested systems under 50 kW being decided more quickly in practice.

Assessment is on cumulative inverter rating across the site, not on battery capacity.

What a retrofit costs in 2026

Installed battery pricing in 2026 runs roughly £400 to £900 per kWh, with the spread driven by brand, coupling, backup specification and how much electrical work the property needs. A retrofit sits inside that band, and often towards the lower half of it because there is no array, no scaffold and no roof work in the price.

For orientation on the products themselves: entry level Fox ESS systems start from around £4,000 installed, Sunsynk systems typically land at £3,500 to £5,000 installed, and a Tesla Powerwall 3 with Backup Gateway is commonly quoted around £7,995 fully fitted. Our 2026 costs guide breaks down where those numbers come from.

The VAT deadline is doing real work here

Battery storage installations carry 0% VAT until 31 March 2027, and that relief explicitly covers retrofit batteries added to existing solar as well as standalone batteries with no solar at all. The extension to standalone and retrofit installations took effect on 1 February 2024.

From 1 April 2027 the rate reverts to 5%. On a typical domestic battery that is worth roughly £225 to £500, which is not the deciding factor on a five figure decision but is a genuine reason not to leave a planned retrofit sitting on a shelf for two years.

What to check on your existing system first

The export question retrofits raise

A battery reduces what you export, because surplus that used to go to the grid now goes into storage. That is normally a good trade, since avoiding a 26.11p purchase beats earning a flat export rate of 12p. It is worth checking the arithmetic on your specific tariff rather than assuming.

There is a second point that specifically affects battery owners. Grid charged electricity discharged back out to the network is not covered by the Smart Export Guarantee obligation. Suppliers may choose to pay for it, but they are not required to and the terms vary by tariff, so confirm before building any plan around exporting stored grid units. Our tariffs guide covers the current export landscape.

One more consideration on sizing

Retrofit sizing is not simply a matter of matching your array. A battery earns from cheap overnight grid units as well as solar surplus, which is why it keeps working through winter when generation collapses. Sizing to summer surplus alone produces a battery that idles for five months of the year. The sizing guide covers the method.

The battery storage guide covers the product landscape, solar panels covers the array side, and every county page in our coverage area carries a local calculator.

VAT position and prices checked 3 August 2026. Clearline gives independent guidance and a free remote design call. Installations are carried out by an MCS-certified installation team for your area.