Hardware · Memo
Round-trip efficiency in plain numbers
Updated
If you put 10 kWh in and get about 9 kWh out, the missing unit is the cost of storage. At a cheap night rate that loss can still be a good trade.
The claim, without the fog
At two similar rates it eats the benefit. Use 88% to 92% as a planning band unless the datasheet for your model says something you can rely on. This is a decision about paperwork, money or safety. Write down the usable kWh, the inverter size, and who services the pack before you compare two prices.
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 rules actually bite on
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.
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.
What I would do next
I would not compare a £5,630 headline with another until usable kWh and the fate of any existing inverter match. Photograph the board, the meter and the wall you have in mind.
Then get two written scopes. If they describe different machines, you do not have two quotes yet.
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.