A home battery only supplies power during a cut if the installation includes backup hardware: an emergency power supply (EPS) socket or circuit, or a backup gateway that isolates the home from the grid. Without it, the battery shuts down with the grid for safety. Tesla's own prices show the cost of the whole-home version: Powerwall 3 is £5,000 without Backup Gateway 2 and £5,975 with it, before VAT and installation. Ofgem's figures work out at an average interruption of about 88 minutes in Great Britain, which a 5 kWh battery covers many times over for the essentials.
Why a battery switches off in a power cut
A battery connected to the grid must not push electricity into the network when the network is down, because engineers working on the lines need them to be dead. So every grid-connected inverter detects the loss of the grid and stops. Unless the installation can separate your home from the grid and run it as a small island, the battery sits full and unused until the power returns.
The three levels of backup
| Level | What stays on | Typical hardware | Things to check |
|---|---|---|---|
| None | Nothing | Standard grid-tied battery | The default on many installations |
| EPS socket or backup circuit | One socket or a few chosen circuits (fridge, lights, router, boiler) | The inverter’s emergency power output, wired to a socket or a small consumer unit | The power limit of the EPS output and how quickly it switches over |
| Whole-home backup | The whole house, within the inverter’s power limit | A backup gateway or changeover switch that isolates the home from the grid | Whether big loads (showers, ovens, EV chargers) are excluded or limited |
Manufacturers publish which configurations support backup. Tesla’s Powerwall 3 provides backup with its Backup Gateway 2; in the configuration that uses a Remote Meter instead, it gives no power in a cut. GivEnergy’s All in One kept the home running during a grid failure only when fitted with its Gateway. Octopus’s published solar-and-battery packages offer whole-home backup only in the Tesla one. Tesla’s own UK prices show what the gateway adds: £5,000 without it and £5,975 with it, before VAT, delivery and installation.
What backup hardware adds to a quote is covered in the solar battery cost guide.
What the installer has to add for backup
Backup is not just a different socket. When the grid fails, the part of your home that stays live becomes an electrical island, and MCS’s battery standard (MIS 3012:2025) sets out what that island needs. In plain terms, for any system that supports island mode:
- An island-mode isolator. All the live conductors and the neutral of the backed-up part of the home must be disconnected from the grid supply when the system switches to island mode. MCS says this can be an isolation relay or a manual changeover switch.
- A neutral-earth bond relay. In island mode the neutral of the island part is connected to earth at one point only, through a relay that makes the connection immediately after the isolator opens and breaks it immediately before the home is reconnected to the grid.
- A suitable earthing arrangement. Protection against shock in island mode must not rely on the earth the network operator provides, so an earth electrode may be needed. MCS prefers a TN-S arrangement in island mode and allows TT where TN-S is not practicable.
That is why quotes list backup separately and why a socket on the side of an inverter is cheaper than a whole-home gateway: the more of the house stays live, the more of this work applies. Ask the installer to say which of these items the price includes.
What a battery can run: power and energy
Two numbers matter in a cut. Power (kW) is how much the battery can supply at once; energy (kWh) is how long it can keep doing it. The backup limits published for popular systems:
| System | Backup power | What the maker says |
|---|---|---|
| Tesla Powerwall 3 with Backup Gateway 2 | Configurable from 3.68 kW to 11.04 kW at 230 V | Backup needs the Gateway; the Remote Meter version gives none |
| Enphase IQ Battery 5P | 3.2 kVA continuous per unit; peaks of 27.8 A for 3 seconds | A Full Backup mode; the other Enphase equipment and wiring decide whether a system has it |
| Fox ESS EVO 10 | Inverter versions from 5 kW to 10 kW | EPS output with switchover under 4 ms and a 200% overload for 10 seconds |
| GivEnergy All in One | 6 kW continuous and 7.2 kW peak off-grid | Whole-house backup needs the GivEnergy Gateway (see the note on GivEnergy) |
The loads that matter are small ones. National Energy Action’s table of typical appliance ratings gives these running powers:
| Appliance | Typical rating | Hours from 10 kWh at that draw |
|---|---|---|
| Phone or tablet charging | 2 to 15 W | 670 hours or more |
| Broadband router | 5 to 15 W | 670 hours or more |
| LED light bulb | 6 to 10 W | 1,000 hours or more |
| LCD television | 25 to 175 W | 57 to 400 hours |
| Fridge-freezer (while the compressor runs) | 100 to 300 W | 33 to 100 hours |
| Microwave | 700 to 1,400 W | 7 to 14 hours |
| Kettle | 2,500 to 3,000 W | 3.3 to 4 hours |
| Electric oven or grill | 2,000 to 2,400 W | 4 to 5 hours |
| Electric shower | 7,000 to 10,500 W | 1 to 1.4 hours |
Adding the top of each range for a router, four LED bulbs, a fridge-freezer, a television and a phone charger gives about 545 W, so a full 5 kWh battery would run that bundle for roughly 9 hours, a 10 kWh battery for about 18 hours and a 13.5 kWh battery for about 25 hours. In practice a fridge-freezer’s compressor cycles on and off, so the real average is lower. Allow about a tenth lost in conversion, in line with this site’s model. Several of the systems above cannot supply the top-end loads at all: an Enphase 5P unit tops out at 3.2 kVA continuous, so a single unit cannot run a 7 kW shower.
How long backup lasts
Divide the energy left in the battery by what you are running:
- Essentials only (fridge-freezer, lights, router, boiler controls): a few hundred watts. A battery with 8 kWh left could run them for most of a day.
- Normal evening use (TV, cooking on a hob, washing): 1 to 3 kW. The same battery lasts a few hours.
- High loads (electric shower, oven, kettle, EV charger): these can exceed the backup output’s power limit altogether, which is why whole-home setups often leave them out.
Two settings matter more than capacity. Backup reserve: many batteries let you keep a percentage back for cuts; a battery that is empty at 7pm because it covered the peak has nothing left for an evening cut. Solar recharge: some backup systems let the panels keep charging the battery while the grid is down, which turns a few hours of backup into days in summer.
A full 13kWh battery runs essentials for more than a day; the table in how long a 13.5kWh battery lasts gives the hours for each load.
How often the power actually goes off
Ofgem publishes each network’s record. Interruptions per 100 customers is how many cuts the average customer has had; minutes lost is the total time without power per customer. Dividing one by the other gives the average length of a single interruption, which is the number backup buyers need. These 2024/25 figures include planned work and largely exclude the biggest storms, so they describe ordinary years, not the worst week.
| Network area | Operator | Interruptions per 100 customers | Minutes lost per customer | Average length of one interruption |
|---|---|---|---|---|
| London | UK Power Networks | 11.2 | 10.5 | 93 minutes |
| North Western England | SP Electricity North West | 26.1 | 27.1 | 104 minutes |
| Cheshire, Merseyside and Northern Wales | SP Energy Networks | 28.6 | 34.5 | 121 minutes |
| Southern Scotland | SP Energy Networks | 31.3 | 25.8 | 82 minutes |
| East Midlands | National Grid Electricity Distribution | 34.3 | 24.6 | 72 minutes |
| Southern England | SSEN | 41.9 | 51.2 | 122 minutes |
| South Wales | National Grid Electricity Distribution | 42.1 | 27.5 | 65 minutes |
| South Eastern England | UK Power Networks | 42.2 | 30.4 | 72 minutes |
| Eastern England | UK Power Networks | 44.6 | 32.5 | 73 minutes |
| West Midlands | National Grid Electricity Distribution | 46.3 | 34.2 | 74 minutes |
| North Eastern England | Northern Powergrid | 51.0 | 46.8 | 92 minutes |
| Yorkshire | Northern Powergrid | 54.7 | 51.2 | 94 minutes |
| South Western England | National Grid Electricity Distribution | 59.0 | 50.4 | 85 minutes |
| Northern Scotland | SSEN | 59.1 | 68.9 | 117 minutes |
| Great Britain | 39.7 | 35.1 | 88 minutes |
Read one way, the table says a London home sees a cut about once every nine years and a North Scotland home about once every 1.7 years. Read another, it says a typical cut lasts between one and two hours. Running the essentials bundle above for 88 minutes takes about 0.8 kWh, which is a sixth of a 5 kWh battery. Backup for an ordinary cut is cheap in energy terms; what costs money is the hardware that makes it possible.
The long events are different, and they are why some buyers choose backup. The day after Storm Eowyn in January 2025, SSEN Distribution reported more than 78,000 of its customers reconnected, about 12,000 households still off and some in its red-warning area expected to be without power until the Monday. Your street may see more or fewer cuts than your network’s average, and the regional pages show the figures for each area.
Is backup worth paying for?
For most homes, backup is insurance rather than a source of savings, so price it separately and decide whether you would use it. It is easiest to justify when:
- someone in the home relies on electrically powered medical equipment or works from home on critical systems;
- the home is on a network with frequent or long interruptions (see the table), or on an overhead rural supply;
- you are already buying a battery that has backup as a modest extra, as with the Powerwall 3’s £975 gateway; or
- you want the home to keep running during the rare multi-day storm outage, which needs a large battery, solar to recharge it, or both.
Questions to ask an installer
- Does this quote include backup? If so, which level: a socket, chosen circuits or the whole home?
- What is the backup output’s power limit in kW, and which appliances are excluded?
- How long does it take to switch over, and will anything reset (clocks, routers, heating controls)?
- Can the solar panels recharge the battery during a cut?
- Can I set a reserve so the battery never empties below a level I choose?
- Which earthing arrangement will the backed-up circuits use in island mode, and does the price include an earth electrode if one is needed?
If backup is the main reason you want a battery, say so in your request. The quote form asks, and installers can size the backup output accordingly. Comparing battery storage quotes with and without backup shows exactly what it costs.
Questions people ask
Does a home battery work in a power cut?
Only if it was installed with backup. A grid-tied battery must stop exporting when the grid fails, so the network is safe for engineers. With an EPS socket, a backup circuit or a whole-home gateway, the battery can keep chosen loads or the whole house running from its stored energy.
How long will a battery power my house in a power cut?
Divide the usable energy left in the battery by the power you draw. A 10 kWh battery with 8 kWh left, running a fridge-freezer, lights, router and phone chargers at about 500 W in total, could last around 16 hours. Kettles, ovens and electric showers drain it in minutes and may exceed the backup's power limit.
What is EPS on a battery?
EPS stands for emergency power supply. It is a dedicated socket or small circuit powered by the battery's inverter during a cut. It is the simplest and cheapest form of backup, but only what is plugged into it or wired to it stays on.
How often are there power cuts in the UK?
Ofgem's data for 2024/25 puts the GB average at about 40 interruptions per 100 customers a year and 35 minutes lost per customer, which is roughly one interruption every 2.5 years lasting about 88 minutes. It varies a lot by network area, from about 11 interruptions per 100 customers in London to about 59 in North Scotland and the South West. Large storms are largely excluded from those figures.
Does backup cost extra?
Yes, in most cases. For the Tesla Powerwall 3, the Backup Gateway 2 is the difference between Tesla's list prices of £5,000 and £5,975, so £975 before VAT and installation. Other systems price EPS outputs or a changeover arrangement as an extra, and the installer must add island-mode isolation and earthing under MCS's battery standard.
Can solar panels work in a power cut?
Ordinary grid-tied solar switches off in a cut. With a battery system designed for backup, and the right gateway or inverter, solar can keep charging the battery while the grid is down. Check the specific equipment, because not every backup setup supports it.
Is backup mandatory with a home battery?
No. Many installations are sold without it. If you do want it, MCS's battery installation standard requires an island-mode isolator, a neutral-earth bond arrangement and a suitable earthing method for the part of the home that stays live.