Retrofit · Site note
Two roof arrays and one battery
Updated
East-west or front-and-rear arrays can feed one battery if the inverter strategy is coherent. Two old inverters plus a new battery is a common muddle.
On the actual building
Ask for a single diagram that shows both strings, both inverters if they remain, and the meter. 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 13.5 kWh and 0 kWp, which is enough to tell two quotes apart if they assumed something else.
The money
Take a 13.5 kWh pack, cycled about 260 times, at 90% round trip. Charge at 13.0p, and value what comes back at 26.0p. Export given up is priced at 4.0p. There are no panels beside this 13.5 kWh pack, so every unit charged comes off the grid. Delivered energy is about 3159 kWh. After charging cost and lost export, the figure saves about £365 a year. Set that against £6,285 fitted, which is a September 2026 illustration at the temporary 0% residential rate, not a quote. That is a simple payback of about 17.2 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 £6,285 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 £6,285 before the DNO route for a 5 kW inverter is known.
If the only place for a 13.5 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 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. £6,285 is a September 2026 illustration for a 13.5 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 13.5 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 £6,285 job, that folder is part of what you bought.
What if the cheap rate is not 7p?
Then the figure changes. This page used 13.0p 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.