About 2,398 solar PV installations are recorded across Merton by DESNZ, at the end of 2025 as first published. That count covers every size, from a terrace roof in South Wimbledon to the borough’s larger commercial arrays, and it says nothing about batteries. But a good share of those systems are household roofs that went up with panels alone, and the most common question we get from their owners is whether a battery can be added now.
Usually it can. Whether it should is a separate question, and it turns on four things: the inverter you already have, how much spare generation there is to store, where a battery would physically go, and the paperwork. For the battery side in full, see our page on battery storage in Wimbledon.
Where Merton’s arrays came from
Merton has a longer solar history than most boroughs. It wrote the Merton Rule in 2003, the first UK planning policy requiring new developments to generate at least 10% of their energy from on-site renewables, and in June 2018 it joined phase two of Solar Together London, the Mayor’s group-buying scheme. Later rounds of that programme included battery storage; earlier installs often did not. If your Wimbledon roof went solar through one of those rounds, or through an installer at around the same time, there is a fair chance it has no battery and an inverter that was not chosen with one in mind.
First: the inverter already on the wall
Every array has an inverter turning the panels’ direct current into the alternating current your home uses. There are two broad ways to add a battery. One is AC coupling: the battery comes with its own inverter and connects alongside the existing one, so the panels are left as they are. The other is to replace the existing inverter with a hybrid unit that manages panels and battery together. AC coupling disturbs less; a hybrid can be neater if the old inverter is near the end of its life anyway. Inverters typically carry shorter warranties than panels, so on an array fitted in the late 2010s in SW19 or SW20, the inverter’s age is worth knowing before anyone quotes.
Second: is there spare generation to store?
A battery added to an array only stores what the house is not already using. PVGIS 5.3 estimates that a 4 kWp array facing due south at 35° in Wimbledon makes about 3,995 kWh a year, with about 480 kWh in June and about 140 kWh in December. That is roughly 16 kWh a day in June against about 4.5 kWh a day in December (our arithmetic, for an unshaded roof). On a June day, after daytime use, there is usually something to put away for the evening. In December there is very little.
That is why a battery on an existing Wimbledon array is sized to the evening, not to winter self-sufficiency. Your own export history is the best guide: if your meter or supplier shows how much you send to the grid each summer month, that is the pool a battery can draw on. The Energy Saving Trust says a typical home system might be 10 kWh but that there is no exact formula, which our guide on what size home battery you need works through.
Third: where it goes in a Grid terrace or a flat
Merton’s housing decides this more than anything. Census 2021 puts 35.7% of the borough’s households in terraced homes, 28.4% in purpose-built flats and 9.2% in flats in converted houses, with semis at 19.0% and detached homes 5.7%. In the Victorian terraces of the Grid and South Wimbledon, there is often no garage and no side access, so the battery goes inside, in a utility space or under the stairs, or on a rear wall. In a flat, the roof array is usually shared or the freeholder’s, so adding a battery means the freeholder’s agreement first.
Wimbledon Village, Wimbledon North and the other conservation areas on Merton’s list do not change the inside of your house, but an outside unit on a wall facing the road is worth raising with Merton’s planning team before it is fixed.
Fourth: the paperwork
Wimbledon’s town centre sits in UK Power Networks’ London Power Networks licence area. Adding a battery changes what is connected, so the installer works out whether panels and battery together stay within the G98 limit of 16 A per phase (3.68 kW), or need a G99 application approved before the work starts. UK Power Networks’ Smart Connect service handles both forms for approved installers and covers battery storage. A new MCS registration and electrical certificate follow the battery installation.
Lifespans and cost
The Energy Saving Trust gives a typical battery lifespan of about 10 to 12 years and puts a 5 kWh system at around £4,600. Panels are designed to last 25 to 30 years. On an array fitted around 2018, a battery added in 2026 reaches that 10-to-12-year mark around 2036 to 2038, well before the panels reach theirs (our arithmetic), so it may need replacing while the array keeps going. The Trust also notes that fitting storage at the same time as panels costs less than a separate visit later; a retrofit is that later visit, though it usually needs no roof work. The VAT is the same either way: 0% on a residential battery, fitted with panels or on its own, until 31 March 2027. For how long the cells themselves last, see how long do solar batteries last.
What we check on a Wimbledon retrofit survey
We look at the inverter, the consumer unit, your export pattern, the space and the network paperwork, and we tell you plainly if the numbers do not justify a battery. We are a NAPIT member (73495), MCS-certified for solar PV and battery storage through NAPIT. The roof side is on our page for solar panels in Wimbledon, and the wider area on our south-west London solar page.
Sources: DESNZ, Renewable electricity by local authority 2014–2025 (published 29 September 2026); European Commission JRC, PVGIS 5.3, PVGIS-SARAH3 2005–2023, 4 kWp, 35°, due south, 14% losses (run 15 October 2026); ONS Census 2021, TS044 (Merton); Merton Council newsroom (Merton Rule, 6 June 2008; Solar Together London, June 2018); Energy Saving Trust, Battery storage (updated 19 August 2026); HMRC, VAT Notice 708/6; UK Power Networks, Smart Connect; Energy Networks Association, EREC G98 Issue 2 (10 March 2025); UK Power Networks and NESO licence boundary data. Read 12–15 October 2026.