Seasons · Seven-day note
Christmas usage and a home battery
Updated
A house full of people, an electric hob and visitors' phone chargers will empty a small battery early. That does not mean the battery is too small for the other fifty weeks.
One ordinary week, not a yearly slogan
Size for ordinary life, and expect festival days to use the grid. I would still write seven days on paper before trusting a savings chart. A grey January week will not look like a bright June week, and a cheap rate only helps during the hours it exists.
Take an 8 kWh pack, cycled about 320 times, at 90% round trip. Charge at 9.0p, and value what comes back at 29.6p. Export given up is priced at 10.0p. About 914 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 2304 kWh. After charging cost and lost export, the figure saves about £461 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 12.2 years on these assumptions. Nudge the night rate up by 4p and see if you still like it.
What the inverter has to do
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.
If a supplier is allowed to move the battery, ask what remains when you leave that supplier or the broadband fails. A schedule kept on the inverter beats a cloud routine you cannot see. At a steady 600 W, 8 kWh is about 13.3 hours.
How the saving gets eaten
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.
A car, a heat pump or the standing charge can flatten the result. The standing charge does not fall because a cabinet arrived. I would not compare a £5,630 headline with another until usable kWh and the fate of any existing inverter match.
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 29.6p 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.