Does a solar battery work in a power cut?

Why do solar panels and batteries switch off in a power cut?

Because the rules for connecting them to the grid require it. The Energy Networks Association's guide says the installer must make sure the equipment will disconnect from the network if your mains supply is interrupted^[1]. Under EREC G98, the standard for small systems in Great Britain, the type tests check that this loss-of-mains protection trips within 0.5 seconds, or up to 1 second for some equipment^[2]. So a standard installation goes off in a power cut along with the street, even on a sunny day.

Backup works by doing the separation first. Tesla describes its backup switch as detecting the power cut and disconnecting the battery from the grid to prevent backfeed, before supplying the home^[3]. G98 itself does not cover a home that runs as an island: such installations are referred to G99, the standard used for larger systems^[2].

What does a battery need to work in a power cut?

Three things, on top of the battery itself.

1. A switch that separates your home from the grid. Each of the four batteries in the table below names a separate unit for backup; some systems build the switch into the inverter, so ask which part does it.
2. Earthing for running on its own. Guidance in the IET's Code of Practice for energy storage, summarised in its Wiring Matters magazine, says a home running on battery alone (island mode) needs its own earth electrode, because the network's earth cannot be relied on, plus switching that disconnects the home and provides a neutral^[7]. A home that already has its own earth electrode may be able to use it, if it meets the requirements^[7].
3. The network operator's agreement. If the battery is meant to run in island mode during a power cut, the faster application route for storage does not apply; the standard G99 process does^[1].

A battery can be installed without backup: Tesla, for example, shows Powerwall 3 fitted with no backup unit at all^[3]. So ask your installer whether yours will work in a power cut, and for how long.

Will a battery back up the whole house or only some circuits?

Either, depending on how it is wired. Tesla's datasheet shows both: whole-home backup, and partial-home backup where essential circuits move to a separate backup board while the rest of the house goes off in a cut^[3]. GivEnergy describes its All In One 2 keeping essential appliances on^[5], and its 6 kW All in One as whole-home backup with a Gateway^[6].

Whole-home backup still has a power limit: the battery's output in kW caps what can run at the same moment. One Enphase IQ Battery 5P supplies 3.2 kVA continuously^[4]. The Centre for Sustainable Energy rates a kettle at 3,000 W^[8], so by our comparison a kettle alone would take nearly all of it. Powerwall 3's datasheet lists output settings from 3.68 kW to 11.04 kW^[3]. Ask which figure your system will have.

How long will a battery run a house in a power cut?

Divide the energy kept for backup by what you run. In our example, a 13.5 kWh battery^[3] keeping half its charge for cuts holds 6.75 kWh. A fridge-freezer, broadband router and TV, at the top of the Centre for Sustainable Energy's ratings^[8], draw 0.61 kW together, so the battery would run them for about 11 hours, by our calculation before losses. That assumes each draws its full rating all the time. A fridge-freezer cycles on and off with its thermostat, so it uses less; a kettle or oven would cut the time sharply. Try your own appliances:

Any share of the battery kept for power cuts is not available to cut your bills day to day, so the reserve is a trade-off you can ask your installer to set. Sizing for everyday use is in what size solar battery do I need.

Is battery backup worth paying for?

Only you can weigh how much a power cut would cost you, but price it as its own decision. The backup switch is usually extra: Tesla's UK price list shows one Powerwall 3 at £5,000 without its Gateway and £5,975 with it, before VAT, delivery and installation^[9]. So ask for backup as a separate line on the quote; our quote check turns missing lines into questions. Whether the battery pays for itself on bills alone is covered in is a solar battery worth it.

If someone relies on powered medical equipment, register for extra help, battery or not. In Great Britain, join the Priority Services Register through your energy supplier or network operator: it is for people who need medical equipment that requires a power supply, among others^[13], and network operators must offer people on it extra help in planned and unplanned power cuts, such as battery packs to keep charging phones or medical equipment^[10]. In Northern Ireland, NIE Networks runs a Medical Customer Care Register for people who rely on electricity for healthcare, with updates by phone during a power cut^[12].

To report a power cut in Great Britain, call 105 for free^[10]; in Northern Ireland, call NIE Networks on 03457 643 643^[11].

What to ask an installer about battery backup

1. Will it work in a power cut?: Will this battery power the home in a power cut, and for how long?
2. The backup hardware: Which part provides backup, and is it in the price as its own line?
3. Whole home or some circuits: Will backup cover the whole home or selected circuits, and which ones?
4. Power and switchover: What can the battery supply in kW during a cut, and how long does the switch take?
5. Earthing: How will the home be earthed while it runs on the battery alone?
6. The network operator: Who applies to the network operator for the backup set-up?
7. The reserve: Can I choose how much of the battery to keep for power cuts?
8. Charging during a cut: Will the solar panels keep charging the battery while the grid is off?