When the grid goes down, what stays on? Compare backup for essential circuits and the whole home, then try a power cut with your own battery and loads.
EPS means Emergency Power Supply. It’s the backup output of a compatible inverter. The installation determines what it feeds. An EPS output can serve a single socket, selected circuits or, with suitable equipment, a whole home.
Keep the fridge, broadband and selected lights running. A dedicated backup socket or a small consumer unit supplies the loads agreed at survey. Other circuits lose power.
A compatible gateway or changeover arrangement separates the home from the grid and supplies the backed-up distribution board. You can use more circuits, within the system’s power limit.
Coverage and runtime are separate. If the same fridge and lights are running, the same battery lasts about as long with either arrangement. Switching on more appliances uses it faster.
Choose what’s backed up, start an outage and switch appliances on or off. This runs entirely in your browser.
Use the manufacturer’s usable battery capacity. Enter charge percentages on that same usable scale. Only enter an additional minimum that still applies during a power cut. Leave it at 0% if the battery’s built-in protection is already accounted for in its usable capacity.
For a steady load, runtime is available kWh divided by battery draw in kW. Battery draw is appliance kW divided by one-way discharge efficiency, plus the inverter’s own battery-side draw. Use a separate idle figure only if the efficiency figure excludes it.
The defaults are illustrative. Enter the continuous AC backup output the inverter and battery can deliver together. The model assumes no solar recharge and excludes brief motor-start surges, temperature effects and ageing. Use measured loads where possible.
Switches set demand. Each row shows whether that circuit has power.
The fridge figure is a running average. The kettle draws 3 kW for one three-minute boil each time you switch it on in whole-home mode. The oven draws its selected wattage continuously until switched off. For cycling appliances, enter a measured average as “Other backed-up load” and check starting power separately.
kWh measures energy stored. kW measures power drawn at once. A bigger battery can run appliances for longer. The inverter and battery must also be able to supply everything you switch on together.
| Average household load | Example demand | Approx. runtime |
|---|---|---|
| 300 W | Light household use | 26 hours 5 minutes |
| 1 kW | Several appliances running | 8 hours 37 minutes |
| 3 kW | Sustained high demand | 2 hours 57 minutes |
We allow for a local earth electrode when planning either type of fixed home backup. It is often a rod, also called an earth spike, installed in the ground. During a power cut, the backed-up installation operates as an electrical island and needs an earthing arrangement that works independently of the electricity network’s earth.
At the survey we check the existing supply and earthing, and identify a suitable location if a new electrode is needed. A home with an existing earth electrode, such as a TT installation, may already have one we can use if inspection and testing confirm it is suitable.
The electrode is one part of the design. We also check how the system disconnects from the grid, how its neutral and earth are referenced in backup mode, and how the protective devices operate. These checks cover both essential-circuit and whole-home backup.
A battery can save money every day and still leave the house without power when the grid fails. Check the handover paperwork for a backup output and a list of the circuits it supplies. If you want to retrofit backup, we check the exact inverter, battery, changeover equipment and consumer unit before confirming the options.
Start with the circuits you want to keep running and their average use. For example, 500 W for 12 hours uses 6 kWh at the appliances. At 90% efficiency with 50 W of inverter draw, that takes 7.27 kWh from the battery. If the cut starts at 80% and 10% must remain, you need about 10.4 kWh of usable capacity. Enter your own figures in the calculator.
Look for usable capacity in the datasheet. If it lists only nominal capacity and an allowed depth of discharge, multiply the two. A 10 kWh nominal battery at 90% depth of discharge gives 9 kWh usable. Enter 9 kWh in the calculator. If the datasheet already says 9 kWh usable, that allowance has already been made.
Your smart meter or energy monitor can show the power in use. For an overnight average, divide the kWh used over the night by the number of hours. For example, 4 kWh in eight hours averages 500 W. Use household consumption from your energy monitor if solar or batteries were supplying some of the load during that period. Enter that as “Other backed-up load” and switch off example appliances already counted. For essential circuits, measure just those appliances. Check the highest simultaneous load separately.
Ordinary grid-connected solar shuts down when the grid fails. A compatible backup system can use solar during an outage if the solar equipment is connected and configured to operate in island mode. Daylight, available battery capacity and the specific inverter all matter.
Changeover can be automatic or manual. Transfer times vary with the inverter and gateway, and some devices may restart. If a device needs uninterrupted power, check its requirements separately. The simulator’s changeover animation is illustrative.
Check their simultaneous demand and any starting surge against the battery and inverter ratings. On a three-phase supply, the backup arrangement and limit per phase matter too. We agree which heavy circuits to exclude or manage at the survey. The example simulator keeps the EV charger grid-only.
It depends on the charge when the cut begins, the usable capacity and the minimum charge the system allows. A backup reserve usually holds some charge back while the grid is available so it can be used during a cut. Only enter it as a discharge floor if your system also prevents you from using it during the outage. A higher reserve leaves less energy for everyday tariff savings.
At the battery’s backup discharge limit, its energy is no longer available to the house. An overload can interrupt the supply much sooner, even with plenty of charge left. Switch off a high-power load and follow the manufacturer’s restart instructions. Automatic restart from sunshine or after the grid returns depends on the equipment.
A compatible battery can charge from the grid before a power cut and run the agreed circuits during it. With no solar or other supported charging source, the energy present when the cut begins must last until the grid returns.
Ask for a quote showing the battery and inverter, any gateway or changeover equipment, alterations to the consumer unit, local earthing, and testing. The cable routes, circuits you want to keep and compatibility of existing equipment determine the price. We confirm these at survey.
Energy Saving Trust · solar during a power cut ↗Manufacturer example · using backup in a power cut ↗
Tell us which circuits matter and how long you want them to run.
