Battery backup: keeping power on when the grid goes down

When the grid fails, a grid-connected inverter must disconnect. To supply the house on its own, the system has to separate your home from the grid first, automatically and reliably, and only then start supplying power. That separation is a safety function built into approved equipment and installed by an electrician. It is not something to improvise with extension leads or a generator plugged into a socket, which can be lethal for the people repairing the network.

An emergency power outlet is the simplest: one socket on the inverter or battery that works during a power cut. You plug in what matters, such as a fridge or a router. Nothing else in the house works.

Backed-up essential circuits are the most common useful setup. The electrician moves chosen circuits, often lighting, the fridge, the router and the boiler or heating controls, onto a separate board that the battery supplies when the grid fails.

Whole-house backup supplies everything, within the battery's power rating. It needs a larger inverter, a switchover device sized for the whole supply and careful thought about heavy loads such as electric showers, ovens and car chargers, which can drain or overload the battery in minutes.

• which circuits are backed up, listed by name
• switchover time, and whether computers or clocks will reset
• whether solar panels keep charging the battery during a long outage
• the maximum power the system can supply, in kW
• how the installer tests the backup at handover

Gas boilers and oil boilers need electricity for their controls and pumps, and heat pumps need a great deal of it. People often buy backup to keep the heating on, then find the heat pump cannot run from the battery or empties it quickly. If heating is the main reason, tell the installer which system you have so it can say honestly what the battery can and cannot do.

Power cuts in most of the UK are infrequent and short. If a few hours without electricity once a year is the concern, a backup-ready system adds cost for little benefit, and a torch, a charged power bank and a plan may be enough. Backup makes more sense in rural areas with overhead lines and repeated outages, for people working from home who cannot lose connectivity, or where someone in the house depends on electrically powered equipment.

If anyone in your household relies on powered equipment, register with your electricity network operator's Priority Services Register as well. A battery is not a substitute for the support that register provides during an outage.

The request form asks what you want to keep running and for how long. It is offered, without your name, to installers in your area; one of them can buy it and then has it alone. We email you who it is before it receives your details. Offrano does not assess the installer's work, so ask for the electrical installation certificate and a demonstration of the switchover before the installer leaves.