Battery-Only Retrofit (Add to Existing Solar): Home battery storage

Adding a battery to solar panels you already have: the single best upgrade for a solar home

If you already have solar panels, especially an older Feed-in Tariff install, there is a good chance you are giving away money every day without realising it. Most solar homes self-consume only 40 to 60 percent of what their panels generate. The rest is exported to the grid for pennies at midday, then bought straight back in the evening at full price. A battery-only retrofit fixes exactly that. It stores your midday surplus so you use it after dark, and it is, for the many UK homes with panels but no storage, the highest-value energy upgrade available.

The appeal is that you are not paying again for the expensive parts. The panels, the roof work and the scaffolding are already done. A retrofit is usually a one-day job, often with no roof access at all, so the cost is far lower than a fresh solar-and-battery install. And because it is battery storage in a home, it qualifies for 0 percent VAT in its own right until 31 March 2027.

Who it suits

This is for the homeowner who fitted solar years ago and watches the export meter tick over at midday while paying peak rates every evening. It is ideal if your self-consumption is low, if your household uses most of its power after the sun sets, and if you want a fast, low-disruption saving. If you have no panels yet, fitting both together is cheaper overall; see solar and battery combined. If panels are not an option for your roof, look at a battery without solar instead.

How it works, and the coupling question

A retrofit battery is usually AC-coupled, meaning it has its own inverter and sits alongside your existing solar inverter. AC coupling is around 90 to 92 percent efficient on the round trip, slightly below the 95 to 97 percent of a DC-coupled new install, but it is the simplest and least invasive way to add storage to a system that already works. The alternative is swapping your existing string inverter for a hybrid one to get DC coupling, which costs more and only makes sense in certain cases; a good installer will tell you honestly which is right for your setup.

There is a useful bonus. Once you have a battery, you can also top it up from a cheap overnight tariff on dull winter days when your panels generate little, so the battery earns its keep all year rather than only in summer.

Realistic cost and payback

A battery-only retrofit typically costs £4,000 to £9,000 including 0 percent VAT, depending on capacity and brand, working out at roughly £500 to £800 per usable kWh installed. Because there is no roof work, it is markedly cheaper than a combined system. Payback usually lands around 7 years, and often faster than that when it is added to an array that is already generating, because you are simply capturing solar you were previously exporting for next to nothing.

A 10 kWh battery cycling once a day tends to save in the region of £550 to £620 a year through better self-consumption and off-peak top-ups. As with any storage, we model the number against your real usage and export rate rather than quote a headline figure, and we will tell you plainly if your export rate is already high enough that a battery makes less sense.

Getting the size right

Size the battery to the daytime surplus you currently export, not to your panel capacity. Look at how much you are exporting in the middle of the day and store that. A typical solar home on 3,000 to 4,500 kWh a year lands on a 10 kWh battery; lower usage suits 5 kWh; high-usage or EV homes go to 13.5 kWh or more. The savings calculator gives a quick first estimate, and always compare quotes on usable kWh, not the nominal headline figure.

Key considerations

• Compatibility: your installer checks that a battery will pair cleanly with your existing inverter and, where needed, whether an AC-coupled unit or a hybrid inverter swap is the better route.
• DNO notification: filed under G98 for systems up to 3.68 kW per phase, or G99 above that, handled by your installer.
• Planning: permitted development for almost all homes, no planning permission, with the usual extra care for listed buildings and conservation areas.
• Safety and warranty: LFP chemistry only, installed to BS 7671 and PAS 63100 by an MCS-registered, RECC-backed installer, warranted for around 6,000 to 10,000 cycles or 10 to 12 years to roughly 70 to 80 percent capacity.
• Manufacturer stability: when choosing a brand, weigh whether the maker will still be around to honour the warranty. GivEnergy entered administration in April 2026, leaving warranty support, firmware and spares for its batteries in real doubt, so we would not recommend a new GivEnergy system and we factor stability into every recommendation. Being independent, we steer you to the brand that fits your home, not the one an installer is tied to.

An illustrative example

As an illustrative composite, not a named customer: a couple in a 4-bed semi had a 6 kWp array fitted in the Feed-in Tariff era but self-consumed only about half of it, exporting the midday surplus for pennies and buying back every evening at full rate. Their annual usage was around 4,200 kWh. They added a 10 kWh usable LFP battery, AC-coupled, in a single day with no roof work, for around £6,500 including 0 percent VAT. In the model, self-consumption rose from roughly 50 percent to about 85 percent, evening imports fell sharply, and with off-peak top-ups on dull days the saving came out near £560 a year, a payback of about 7.2 years. Any true surplus is still exported for Smart Export Guarantee income on top. These figures are illustrative and depend on your array, usage and export rate.

Ready to stop giving your solar away? Read the cost guide, run the savings calculator, or get a straight quote from MCS-registered installers. The grants and funding page covers 0 percent VAT and the Smart Export Guarantee, and the FAQs answer the common retrofit questions. You can also compare a 10 kWh system or step up to a 13.5 kWh setup.