Backup · Site note
What a home battery does in a power cut
Updated
Without a gateway and an islanding design, it turns off. With a gateway, it can supply the circuits that were put on the backup side, for a limited number of hours.
On the actual building
Solar may recharge it during the cut only if the system is designed to do that. Most basic installs are not. The cupboard, the supply and the tariff decide it. A national package price skips all three.
In most single-phase houses in Britain, 3.6 kW is in the band where G98 connect-and-notify is often enough. Export can still be capped. Agree that before the inverter is ordered. The figure uses 5 kWh and 0 kWp, which is enough to tell two quotes apart if they assumed something else.
The money
Take a 5 kWh pack, cycled about 280 times, at 90% round trip. Charge at 7.0p, and value what comes back at 27.2p. Export given up is priced at 6.0p. There are no panels beside this 5 kWh pack, so every unit charged comes off the grid. Delivered energy is about 1260 kWh. After charging cost and lost export, the figure saves about £245 a year. Set that against £3,650 fitted, which is a September 2026 illustration at the temporary 0% residential rate, not a quote. That is a simple payback of about 14.9 years on these assumptions. 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 £3,650 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 sign off a 5 kWh install from a video call. Someone has to open the consumer unit.
If the only place for a 5 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 G98 note for this 3.6 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. £3,650 is a September 2026 illustration for a 5 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 5 kWh example the inverter is 3.6 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 £3,650 job, that folder is part of what you bought.
What if the cheap rate is not 7p?
Then the figure changes. This page used 7.0p as a charge rate and 27.2p 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.