Adding a battery to a 2010 solar system without losing the FIT

Sixteen years of Feed-in Tariff payments still to run, and nothing was going to be allowed to jeopardise them.

A Fox ESS hybrid inverter and battery fitted beside an older solar installation in Milton Keynes

Quick facts

System: Battery retrofit to existing solar
Existing array: 3.96 kWp — 18 × Sharp ND-220, fitted around 2010
Inverter: Fox ESS H1-6.0-E-G2-WL, 6 kW hybrid (replacing the original)
Battery: 2 × Fox ESS EP12 — 23.04 kWh nominal, 20.74 kWh usable
Backup: None — the customer did not want backup
EV charger: None
Property: House built in the 1970s–80s
Installed: July 2026, with the second battery added in September
Estimated savings: £669 a year
Estimated payback: 5 years

Estimated at quote stage based on the customer’s usage and tariff at the time. Actual savings vary.

Why they got in touch

A homeowner in Milton Keynes had a solar array from around 2010 and roughly twelve years of Feed-in Tariff payments still to come. The brief was specific: “improve self-consumption… reduce daytime grid import… optimise use of cheap night-rate electricity… realistic ROI of around 6–7 years.”

What we recommended and why

The technical answer was easy: replace the ageing inverter with a hybrid and put a battery behind it. The complication was the Feed-in Tariff.

FIT payments on a system this old are made on deemed export — you are paid for an assumed 50% of generation, whether or not you actually export it. Self-consuming more does not reduce that payment, but changing the system can put the tariff at risk if it is handled wrongly. MCS could not reissue a certificate for the 2010 Sharp panels, which is the usual route. We worked it through against Ofgem's FIT guidance instead, and the tariff was preserved.

Our calculator put the first EP12 at roughly £669 a year saved, with about a five-year payback.

The install

A day for the inverter swap and the first battery. The customer came back in September and we added a second EP12 with a junction box, doubling the storage to 23.04 kWh.

The result

A sixteen-year-old array now charging a 23 kWh battery on the overnight rate, with the Feed-in Tariff intact.

“The system is functioning well… replenishing the battery on night-time cheap rate from 1am to 4am and we are mostly self-consuming… Our daily electricity charges have almost disappeared!”
Homeowner, Milton Keynes

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