Retrofit · Site note
Adding a battery to an existing 4 kWp array
Updated
A 4 kWp array is a very common UK system. In summer it can fill a 5 to 10 kWh battery and still export.
On the actual building
In winter it will not. The battery size should follow the evening load and the cheap-rate window, with the 4 kWp figure used as a cap on how much solar charging you can expect, not as a target to match in kilowatt-hours. The cupboard, the supply and the tariff decide it. A national package price skips all three.
In most single-phase houses in Britain, 5 kW sits above the 3.68 kW G98 line, so this wants a G99 application before anyone energises it. Export can still be capped. Agree that before the inverter is ordered. The figure uses 8 kWh and 2.5 kWp, which is enough to tell two quotes apart if they assumed something else.
The money
Take an 8 kWh pack, cycled about 260 times, at 90% round trip. Charge at 9.0p, and value what comes back at 26.0p. Export given up is priced at 4.0p. About 743 kWh a year of the charge is treated as spare solar from 2.5 kWp that might otherwise have been exported. Delivered energy is about 1872 kWh. After charging cost and lost export, the figure saves about £343 a year. Set that against £5,630 fitted, which is a September 2026 illustration at the temporary 0% residential rate, not a quote. That is a simple payback of about 16.4 years, which is past a typical 10-year warranty. Nudge the night rate up by 4p and see if you still like it.
Where the job is a qualifying install in a home, the 0% rate runs to 31 March 2027 and is due to become 5% after that. This figure prices the work at £5,630 on that basis. It does not apply to a shop, and it does not apply to a battery you buy and fit yourself.
When to stop
I would not pay the bulk of £5,630 before the DNO route for a 5 kW inverter is known.
If the only place for a 8 kWh cabinet is an escape route, a hot boiler cupboard or an unapproved balcony, stopping is a successful survey.
Before a deposit, I would want usable capacity, the warranty counterparty and a G99 note for this 5 kW inverter. The cupboard has to be a place you can stand in, at a temperature the datasheet allows. A five-minute video is not that.
Common questions
Is the price on this page a quote?
No. £5,630 is a September 2026 illustration for a 8 kWh example at the temporary 0% residential rate. Your survey replaces it.
Will it stay on in a power cut?
Only if a gateway or essential-load board was designed and paid for. A standard grid-tied battery turns off with the grid. On this 8 kWh example the inverter is 5 kW, and backup is still a separate line.
What paperwork should I keep?
Keep the MCS certificate where the job is certified, the electrical certificate, the DNO record, the schematic, the warranty registration and a photo of the schedule. For an illustrated £5,630 job, that folder is part of what you bought.
What if the cheap rate is not 7p?
Then the figure changes. This page used 9.0p as a charge rate and 26.0p as the rate avoided. A smaller gap means a worse payback. Read the live tariff.
Sources
Want a price for your house?
The figures above are a rough guide. An installer still needs to see your fuse board, where the battery will go, and whether you want power during a blackout.