Battery retrofit: adding storage to a house you already live in

If you already have solar panels, the battery stores what you would otherwise export in the afternoon. The design question is how the battery joins the existing system, and whether the old inverter is worth keeping.

If you have no panels, the battery charges from the grid, normally on a cheap overnight or time-of-use tariff, and runs the house through the expensive hours. Here the tariff does all the work. Before you commit, check which tariffs your supplier offers for battery owners, whether you need a smart meter first and how long any tariff is fixed for. A battery bought for one tariff can look very different if that tariff is withdrawn.

A retrofit survey is short but should not be skipped or done purely from photos. The installer needs to see the consumer unit, the meter position, the main earthing arrangement, the existing solar inverter if there is one, and the route a cable would take to the place where the battery could go.

It is also the moment to spot existing problems: an old fuse board without RCD protection, a damaged cut-out or signs of overheating. An installer cannot simply connect new equipment to an installation it has doubts about, so some retrofits start with electrical work nobody had planned for. That is better discovered at the survey than on the day.

• make and age of the existing inverter, and its remaining warranty
• space and a spare way in the consumer unit, or the need for a new one
• earthing arrangement and condition of the existing wiring
• mobile signal or Wi-Fi where the battery will be, for monitoring
• your half-hourly usage from the smart meter, if you have one
• any existing export arrangement, such as the Feed-in Tariff or a Smart Export Guarantee tariff

An AC-coupled battery comes with its own inverter and leaves your solar inverter as it is. That keeps the retrofit simple and does not touch the panels' warranty, but you pay for two inverters. A hybrid inverter that runs both panels and battery is neater, yet replacing a working inverter only makes sense if it is old or already failing.

Ask whoever quotes to say plainly which option they propose and why, and what happens to the manufacturer's warranty on the existing equipment if someone other than the original installer changes the wiring.

Many solar systems fitted before 2019 are still on the Feed-in Tariff, where export may be deemed rather than metered. A battery should not be allowed to charge from the grid and discharge into a system whose generation meter then records it as solar output. Installers deal with this in the way the battery is wired and configured. If you are on the Feed-in Tariff, mention it in your request and ask your FIT licensee what it needs to be told.

Source: gov.uk · ofgem.gov.uk

In a new build the battery position can be planned. In an existing house, the obvious cupboard is often the wrong place. The fire safety standard for home batteries, PAS 63100, restricts where a battery may go, and an installer following it may rule out a loft, an understairs cupboard on the escape route or a bedroom. Garages and external walls are common alternatives.

Moving the battery further from the consumer unit means longer cable runs and more making good, which is part of the price. Ask for the proposed route to be marked up so there are no surprises about chasing walls or cables across the outside of the house.

You describe the house, the existing system and the tariff you have in mind, and the request is offered without your name to battery installers who cover your area. Only one of them can buy it. We send you that installer's name, address and phone number by email before it sees yours. The service costs you nothing, and there is no obligation to accept a quote. Offrano checks that buyers are registered businesses, not whether they are MCS certified, so check that yourself on the MCS website.