A power cut quickly shows which parts of a property matter most. Heating controls, refrigeration, lighting, broadband, alarms and medical equipment can all be affected at once. A full house backup battery is designed to keep your property operating during an outage, giving you far more than the limited emergency socket found on many standard battery systems.
For households and businesses across London and the Home Counties, the aim is simple: maintain safe, useful power when the grid is unavailable, while also storing lower-cost electricity or surplus solar generation for everyday use. The right system needs careful design. Battery size matters, but so do the inverter, the way your consumer unit is configured and the appliances you expect to run.
What does a full house backup battery do?
A full house backup system uses battery storage and a compatible inverter to supply the property when the incoming grid supply fails. It detects the outage, safely disconnects from the network and changes to battery power. This changeover is automatic on suitably specified systems, so there is no need to find extension leads or start a generator in poor weather.
The key difference is in the scope of backup. An essential-load system supplies only selected circuits, such as lighting, the boiler, fridge freezer, internet router and a few sockets. That is often a sensible and cost-effective option.
A full house backup battery is configured to support the whole property supply. It can keep the normal circuits live, subject to the available battery capacity and inverter output. It does not mean every high-demand appliance can run at the same time indefinitely. An electric shower, induction hob, heat pump, immersion heater and EV charger can each place substantial demand on the system. Good design gives you a practical plan for managing these loads rather than an unrealistic promise of unlimited off-grid power.
Full house backup battery or essential-load backup?
The right answer depends on how you use your property and what you need to protect. For a flat or smaller home, backing up essential circuits may provide excellent peace of mind at a lower installation cost. It keeps the basics running through most short power cuts without needing a large inverter or battery bank.
Full-property backup is often more appropriate for larger homes, properties with home offices, electrically controlled heating, security systems or people who cannot tolerate interruptions. It can also suit small businesses where lost power means lost trading, spoiled stock, interrupted communications or inaccessible security equipment.
There is a middle ground too. A system can be designed for whole-property changeover while using load controls to prevent selected heavy loads from operating during an outage. For example, the EV charger or immersion heater may pause automatically, leaving more power available for the rest of the building. This approach can provide broad coverage without oversizing the battery purely for occasional peak demand.
Capacity and power are different things
Battery capacity is measured in kilowatt-hours, or kWh. It indicates how much energy is stored and therefore how long the battery may last. Inverter power is measured in kilowatts, or kW. It determines how much electricity the system can deliver at one time.
Both figures matter. A battery with plenty of kWh may still struggle if the inverter cannot meet the starting demand of several appliances. Equally, a powerful inverter will not provide an all-night supply if the battery is too small for the loads being used.
A typical evening of lighting, refrigeration, broadband, television and a gas-boiler control system uses far less power than cooking electrically or charging a car. Before specifying equipment, an experienced installer should look at your consumption patterns, large appliances, heating arrangement, existing solar PV and the length of backup you want. There is no one-size-fits-all battery size.
How long will backup power last?
The honest answer is that it depends on the load. During a short outage, a sensibly sized battery can allow a home to continue operating with little change. During an extended outage, managing demand becomes more important.
Consider a battery with 10 kWh of usable stored energy. If the property is using an average of 500 watts, it could provide roughly 20 hours of supply before allowing for system losses. At an average of 2 kW, that same stored energy may last around five hours. Turn on an electric oven, tumble dryer or 7 kW EV charger, and the available runtime falls sharply.
Solar can extend backup time during daylight, provided your solar PV and battery system are designed to operate together while isolated from the grid. This is not automatic. Some solar installations shut down during a power cut because they are not equipped for island-mode operation. A full house backup design needs the correct inverter and protection arrangement so that solar generation can continue safely and charge the battery where conditions allow.
Winter is another consideration. Solar output is lower on short, overcast days, which means battery sizing should not rely on summer generation alone. We discuss realistic outage scenarios with customers, including how the property will be used after dark and during colder months.
What a safe installation involves
Whole-property backup is not a battery placed beside an existing consumer unit. It is an electrical installation that must be planned around the property, its supply characteristics and the selected equipment.
The system needs a safe means of isolating from the grid. This prevents electricity from feeding back onto network lines during an outage, protecting utility engineers and meeting the required technical standards. The backup equipment, earthing arrangement, protective devices and cable routes must all be assessed as part of the design.
Your existing consumer unit may be suitable, or it may need upgrading. Older boards, limited spare ways, unsuitable protective devices or previous alterations can affect the project. In some properties, it is better to install a dedicated backup arrangement or make improvements to the wider electrical installation first. That is not unnecessary extra work. It is how reliable backup is achieved without compromising safety.
Supply type matters as well. Most homes have a single-phase supply, while some larger homes and commercial premises have three-phase power. A three-phase full house backup battery requires particular attention because the loads may be spread unevenly across phases. The proposed system should be designed for the actual supply rather than assumed from the size of the building.
Making the system work in everyday life
Backup is only one benefit. For most of the year, the battery can store surplus solar electricity for use later in the day, reducing reliance on imported power. Where suitable tariffs are available, it may also charge at lower-cost times and support the property when electricity prices are higher.
The controls should be straightforward. You should be able to see battery state of charge, solar generation and household demand through the approved monitoring platform, without needing to become an energy engineer. More importantly, the system should be set up with sensible priorities: maintaining an appropriate reserve for a possible outage, if that is your preference, rather than using every stored unit for daily savings.
There is always a trade-off. Holding a larger emergency reserve gives stronger resilience but can reduce the energy available for tariff savings each day. Using more of the battery routinely may improve day-to-day savings but leave less stored power when a fault occurs. The right setting depends on your property, local outage concerns and priorities.
Questions to settle before you choose
Before investing, decide what “full house” means for your household or business. Do you need every circuit available, or do you need uninterrupted essentials plus selected high-value equipment? How long would a power cut need to last before it causes a genuine problem? Are you prepared to avoid energy-hungry appliances while running on battery power?
It is also worth considering future plans. A new heat pump, induction cooking, electric vehicle or home extension can change electricity demand considerably. Specifying a scalable battery and allowing for future electrical work can be more economical than starting again later.
Home EESS approaches these projects as complete electrical systems, not standalone products. A site survey and clear discussion of your loads, solar plans and resilience expectations give you a system that is properly matched to the property – with quality installation and support after the work is complete.
A well-designed backup battery should make a power cut less disruptive, not give you another system to worry about. Start with the appliances and routines you need to protect, then let the electrical design follow from there.
