A battery to move energy from one time to another
A 9 kWh battery at the back of the house stores energy so it can be used later than it was made. In winter it is filled overnight on cheap electricity; for the rest of the year the sun does more of the work.
What is in the yard
Against the rear wall, to the left of the back door as seen from the yard, stands a SolaX battery under a quilted silver insulation blanket, with its inverter on the wall above. It was fitted in two stages: 6 kWh in July 2023, for £6,150, and a further 3 kWh in October 2024, bundled with the EV charger for £2,335 between them. There was no grant and no VAT relief. In all it is 9 kWh, of which about 8 kWh is usable, because the batteries are not allowed to drop below 10 percent. Much of the household's own money for its retrofit measures came from the solar panels' Feed-in Tariff, described in the solar story.
How the energy moves
There are two inverters. The one in the loft turns the solar panels' output into the current the house uses. The one beside the battery looks after the battery, charging it and sending energy back. Both connect to the house's electrics, so the house, the panels, the battery and the grid all share one supply. The house uses what it needs first. The battery can take in energy from the panels or the grid at one time and give it back later, and the grid fills any gap.
How it is run
The household changes the battery's work mode with the year and the weather. In general, from November to March the batteries are filled overnight at the cheap rate, on Octopus's Intelligent Go tariff. For the rest of the year the sun does more of the work. In 2025 the battery charged roughly 1,810 kWh and gave back roughly 1,770 kWh, read by eye from the SolaX app's chart, so both are approximate.
What shifting energy is worth
Intelligent Octopus Go charges 7p a unit between 11:30pm and 5:30am, and the homeowner puts its rate for the rest of the day at about 28p (published figures for 2025 were about 25 to 27p, and they vary by region). Every unit bought overnight and used later, instead of bought at the day rate, saves about 21p. The battery gave back roughly 1,770 kWh in 2025, so if all of that had otherwise been bought at the day rate, it was worth up to about £370 over the year. That is an upper figure and an illustration of scale, not a saving anyone has measured.
Limits and trade-offs
Batteries are not allowed to drop below 10 percent, so about 8 kWh of the 9 is usable. The batteries also stop working at freezing temperatures. Freezing is uncommon on the south coast of England but does happen, so the household added an insulation blanket, a cheap, low-tech fix. In very cold weather the heat pump has sometimes used up the whole battery and then drawn from the grid. The app does not show how much of the battery's charge came from the panels and how much from the grid, so no split is claimed here.
Reading the SolaX app
The app's overview shows a "Yield" of 3,395.60 kWh for 2025, about double what the panels made. The figures suggest it adds the battery inverter's output to the panels', which would count some energy twice. The app does not say so, so that is an inference. The panels' own output was roughly 1,750 kWh.
Benefits
- Buying cheap and using later: energy bought overnight at the cheap rate (published at about 7p a unit in January 2025) can be used later, when the same tariff's rate outside the cheap hours was about 28p by the homeowner's account (published figures for 2025 were about 25 to 27p). On that basis the 1,770 kWh the battery gave back in 2025 was worth up to about £370.
- Energy made or bought at one time can be used at another.
- Cheap overnight electricity can fill the battery in winter, when the panels make little.
- In the household's own experience the battery and the panels work well together.
Limitations
- This is one house's set-up and habits: the work mode changes with the year and the weather.
- The 2025 figures are read by eye from the app's chart and are not meter readings.
- How much of the battery's charge came from the panels, and how much from the grid, is not recorded.
- Nothing here is a promise of saving or payback for another home.
- The 8 kWh usable figure is worked out from the 10 percent floor, not read from a label.
- The £370 is an upper figure: it assumes everything the battery gave back would otherwise have been bought at about 28p, and the 28p is the homeowner's figure, not a published one.