Money · Cost note
Separate payback from the value of backup
Updated
If you want the freezer and the router to survive a cut, you may rationally buy a battery that fails a pure payback test. Say that out loud.
The figure, and what it leaves out
Do not launder a backup preference through an unrealistic savings chart. People still want a number. A fair illustration for this 5 kWh sketch is £3,650 fitted at the temporary 0% residential rate, before a new board, scaffold, a long cable or a gateway.
Take a 5 kWh pack, cycled about 340 times, at 90% round trip. Charge at 7.0p, and value what comes back at 30.8p. Export given up is priced at 12.0p. There are no panels beside this 5 kWh pack, so every unit charged comes off the grid. Delivered energy is about 1530 kWh. After charging cost and lost export, the sketch saves about £352 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 10.4 years on these assumptions. Nudge the night rate up by 4p and see if you still like it.
Lines that move the total
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 sketch uses 5 kWh and 0 kWp, which is enough to tell two quotes apart if they assumed something else.
A consumer unit with no spare way is extra. A gateway is extra, and only if backup was the brief. Exclusions are fine when they are written down. Hidden ones are why two quotes sit thousands apart.
A number I would not chase
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 sketch 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.
I would not sign off a 5 kWh install from a video call. Someone has to open the consumer unit. Price per kWh is a blunt tool. Put inverter kilowatts and the name that honours the warranty back into the comparison.
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 sketch 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 sketch changes. This page used 7.0p as a charge rate and 30.8p 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.