Money · Cost note
Price per kWh of battery is a blunt comparison
Updated
A cheaper cell behind a smaller inverter, with no backup and a vague warranty, can look better per kWh and be the worse system. Divide by usable kWh if you must, then put inverter power, warranty holder and backup back into the comparison.
The figure, and what it leaves out
The opening is the claim. What follows is how I would test it against a bill, a cupboard and a connection form. People still want a number. A fair illustration for this 8 kWh example is £5,630 fitted at the temporary 0% residential rate, before a new board, scaffold, a long cable or a gateway.
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.
Lines that move the total
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.
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 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.
I would not pay the bulk of £5,630 before the DNO route for a 5 kW inverter is known. 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 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.