Tariffs · Seven-day note
Agile Octopus and home battery arbitrage
Updated
Agile prices electricity every half hour. A battery can charge in the dips and avoid the spikes, which is the strongest theoretical arbitrage and the easiest way to be disappointed if nobody is actually automating it.
One ordinary week, not a yearly slogan
You need an inverter that can follow a schedule, and a household that can tolerate occasional expensive days. 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 a 13.5 kWh pack, cycled about 300 times, at 90% round trip. Charge at 13.0p, and value what comes back at 28.4p. Export given up is priced at 8.0p. About 2835 kWh a year of the charge is treated as spare solar from 6 kWp that might otherwise have been exported. Delivered energy is about 3645 kWh. After charging cost and lost export, the sketch saves about £696 a year. Set that against £8,805 fitted, which is a September 2026 illustration at the temporary 0% residential rate, not a quote. That is a simple payback of about 12.7 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 sketch uses 13.5 kWh and 6 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, 13.5 kWh is about 22.5 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 sketch prices the work at £8,805 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 pay the bulk of £8,805 before the DNO route for a 5 kW inverter is known.
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. £8,805 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 sketch 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 £8,805 job, that folder is part of what you bought.
What if the cheap rate is not 7p?
Then the sketch changes. This page used 13.0p as a charge rate and 28.4p 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.