What we recommend · Power cuts
Will a home battery keep the house running in a power cut?
Only if it's wired for it. Here's what that takes, and how long it lasts.
Illustrative figures. The hours and prices on this page are worked examples, not confirmed HES figures. Installed prices are pending — HES is a new business, and prices will be confirmed once our installer partner's rate card is agreed.
Hours a full 20 kWh battery runs a home through a power cut — about 45 on the essentials, about 12 running the whole house
The essentials — lights, fridge and freezer, router, and the heating's pump and controls — draw around 400 W together, so 18 kWh usable lasts roughly 45 hours on the essentials and about 12 running the whole house. A battery bought for the bill is fullest at dawn and emptiest by late evening, so a reserve matters — see below. The EV charger, the hot tub heater and an electric shower wait for the grid: the inverter delivers 3.68 kW on a 20 kWh system and 6 kW on a 30 kWh one, and those are the first loads to exceed it. Without solar, nothing refills the battery while the grid is down. Illustrative worked example, 13 September 2026. Your installer partner's inverter sets the real limit, and your recommendation carries your own hours.
The evening it happens
The street goes dark. Yours doesn't.
It is a Tuesday in January, ten to seven. The lights dip once and the road outside goes black — no streetlamp, no glow from next door, a car alarm starting up somewhere down the lane. In your kitchen the lights are still on, the router is still green and the fridge is still humming. The only clue is how quiet it has got.
The battery beside the house noticed the grid drop before you did. It cut the house loose from the network and picked up the load in less time than it takes a bulb to flicker, and it will hand back to the grid on its own when the grid returns. You find out from the neighbours in the morning. How long it carries on depends on what was left in it at ten to seven, which is why we ask whether you want some held back.
Night in the window. The lamp, the router and the fridge carry on.
What stays on
A day and a half to two days of the things that matter
About 45 hours is close to two nights and the day between, on a full 20 kWh battery running the essentials. That is enough to see out anything the network fixes in a working day and most of what a storm does, and it is more than double what a 13.5 kWh battery gives you, which is the size most people are sold. Nothing refills it while the grid is down, so what you choose to run decides what you get.
| Running | Draw | 20 kWh lasts |
|---|---|---|
| Lights through the house | 60 W | |
| Fridge and freezer | 120 W | |
| Router, phones on charge | 20 W | |
| Heating pump and controls | 80 W | |
| Television and a laptop | 120 W | |
| The essentials together | 400 W | about 45 hours |
| Add a heat pump on a cold night | 2,900 W | about 5 hours, added to the essentials |
Everything drawn here is on.
What waits
The car and the hot tub sit this one out
The inverter sets the limit: 3.68 kW on a 20 kWh system, 6 kW on a 30 kWh one. That ceiling is the honest limit of any home battery, whatever the brochure implies. A 7 kW car charger takes most of it on its own. A hot tub heater draws 3 kW. An electric shower draws 9. Whole-home backup keeps the house running; it does not turn the drive into a filling station.
That matters less than it sounds. The time-of-use tariff already fills the car every night for the cheap rate, so on the evening the street goes dark the car is full, the house is lit, and the hot tub is warm under its cover until morning. What you give up in a cut is the top-up, not the thing.
The window is lit. The car and the tub wait for the grid.
The choice
Three ways to specify it
For most homes the middle option is the right one. It covers what a two-hour cut actually threatens — a dark house, a warming freezer, a dead router on a working day — for a fraction of the gateway price, and it leaves the big loads to wait, which on a domestic inverter they would have to anyway. The first column is doing nothing about backup, and it is a legitimate choice: the battery pays for itself on the bill whichever column you pick.
| Option | Bill only | Essentials circuit | Whole home |
|---|---|---|---|
| Stays on in a cut | Nothing — the house goes dark with the street | Lights, fridge and freezer, router, heating pump and controls, one ring of sockets | Everything the inverter can carry at once |
| Waits for the grid | Everything | EV charger, hot tub, oven, showers, the rest of the sockets | EV charger and the hot tub heater, on most inverters |
| Extra kit | None | Backup fuse board, earth rod | Gateway with transfer switch, earth rod |
| Hours from a full 20 kWh | — | about 45 | about 12 |
| Power available in a cut | — | 3.68 kW (20 kWh) or 6 kW (30 kWh) | 3.68 kW (20 kWh) or 6 kW (30 kWh) |
| Added to the installed price | £0 | Pricing pending | Pricing pending |
The mechanism
A battery only carries the house if it is wired to
A standard home battery shuts down in a power cut, for the same reason a solar system does: the rules require anything on your side of the meter to stop feeding a dead network. To keep running, the system has to isolate the house from the grid first and only then supply it. That switch is either built into the inverter or fitted as a separate gateway between the meter and the fuse board (your consumer unit).
Either way the house needs its own earth rod, because in a cut the network's earth goes with it. That is why backup costs more than a setting change, and why the inverter should be chosen with it in mind even if you leave it unwired for now.
Three circuits move to a backup board. The rod goes into the ground.
The objection
Power cuts are rare, so who is this for
Most homes will barely use it, and we would rather say so here than after the survey. If your experience of the grid is a flicker every couple of years, buy the battery for the bill and treat backup as a small option rather than a reason.
The average home in Great Britain has about 0.4 unplanned power cuts a year and is off supply for around 38 minutes in total — roughly one cut every two and a half years, usually short.
The people it is for know who they are. Overhead lines at the end of a lane. A private water supply on an electric pump. Tell us the pump's rating — motors draw several times their running power for a moment when they start, and the installer checks the inverter can take it. Electric heating or a heat pump, so a cut in January is a cold house. A freezer full of food, a home office with a deadline, medical equipment, electric gates. For those homes the essentials circuit is a modest addition to a purchase the bill has already justified.
Storm Arwen, November 2021: over a million homes lost power. About 40,000 were off for more than three days and nearly 4,000 for over a week.
A battery alone will not cover a week. Without solar nothing refills it during a cut, so a 20 kWh battery is two nights of essentials, not a fortnight. A week without power is a generator question, and if you live where that happens we would say so.
We work out whether a battery pays for your house and, if it does, introduce you to an installer. We do not install anything. The installer pays us a percentage of the installed price when the work is finished. You pay the installer directly and never pay us. Backup adds to the installed price, so it adds to what we earn. That is why it is priced as its own line and you can leave it off.
The method
Your hours come from your bill
The battery is sized on what your house uses, not on the power cut. So the estimate that tells you the saving is the same one that tells you the hours — and it starts with four questions, none of them your name.
Check your savings
Four questions size the battery from your monthly spend, any car charged at home, the big loads in the house and whether anyone is home during the day. The estimate shows the saving and the size; the size sets the hours.
Send us your bill, meter and fuse board
Say which circuits have to stay on in a cut. The fuse board photo shows us how they are split and what can move to a backup board.
Your recommendation prices backup as its own line
The battery is justified on the bill alone. Backup sits beneath it with its own cost and its own hours, so you can take it or leave it without moving the payback.
The survey confirms the earthing and the circuits
The installer partner checks where an earth rod can go and which circuits move. That is what the survey is for.
Before they leave, have them prove it
Ask the installer to turn off the main switch and show you the house stay on. A backup that has never been tested is a promise, not a feature.
What the installer will ask you
- Where the fuse board is and whether there is wall space beside it for a second board.
- Whether there is open ground near the house for an earth rod. A patio can be lifted; a fully paved plot is harder.
- Which of the big loads — EV charger, hot tub, shower — you would accept losing in a cut.
- Whether anyone in the house relies on powered medical equipment.
Questions people ask about backup
Will my EV charge during a cut?
No, in almost every case. A 7 kW charger exceeds what the inverter can deliver alongside the house, and a gateway drops that circuit first. If the car matters in a cut, keep it charged overnight — the time-of-use tariff already rewards that.
Will the heat pump run?
Usually, on whole-home backup, and it is the load that shortens the hours most. A heat pump drawing 2 to 3 kW on a winter evening turns 12 hours into closer to 5. Your recommendation would say so rather than let you find out.
Does the battery keep some charge back for a cut?
If you ask it to. Most systems let you set a reserve the battery will not discharge below. Holding 20 per cent back on a 20 kWh battery keeps about 4 kWh for a cut and gives up about 4 kWh of cheap-rate switching every day, which shows up in your saving. Both numbers go in your recommendation so you can choose.
Do I need solar for this to work?
No, to carry a cut. Yes, to carry a long one — solar is the only thing that refills a battery while the grid is down. Nothing on the roof stays our default; if you live somewhere that loses power for days at a time, we would tell you that changes the answer.
Is it automatic?
Yes. The gateway or inverter detects the loss of grid and switches within a fraction of a second. Lights may flicker and the oven clock may reset. When the grid returns it reconnects on its own.
Can it be added later?
Often, if the inverter supports it — which is why we specify one that does even when backup is left off. The earth rod and the extra board are the installer's work on a second visit, and cost more that way than at the first.
Read next
More on power cuts
Did You Know Your Energy Company Can Now Cut Off Your Power for 3 Hours a Week?
Since August 2026 the grid operator can schedule rotating power cuts without Government permission. A battery sized for everyday savings, not sold as a backup gadget, can mean you don't notice when the lights go out.
How long would your house run?
Four questions size the battery on your bill, and the size sets the hours. No contact details until there is a figure to send you, and the workings beside it when there is.