A backup battery is useful only if it can deliver enough power to start your appliances and enough energy to keep them running. Here is how to estimate both, without buying more than you need.
Choose the essentials first
Start with the items you really want during a power cut: perhaps a phone, router, a few lamps and a fridge. Write down each item's running wattage and the number of hours it needs power.
For a fridge or freezer, the motor switches on and off, so its average consumption is often much less than its running rating. Use an energy monitor or the appliance's stated daily kWh consumption for an estimate.
- Lighting and phone charging are usually small loads.
- Broadband may still fail if local network equipment loses power.
- Heating controls and pumps can need more than a simple USB battery.
Power (W) and energy (Wh) are different
Power in watts tells you how demanding an appliance is at a moment in time. Energy in watt-hours is power multiplied by time. A steady 50 W load running for ten hours uses about 500 Wh.
A 1,000 Wh power station will not normally provide a full 1,000 Wh to mains appliances. Inverter losses, battery reserves and standby draw reduce usable output.
Use the runtime calculator with a sensible efficiency allowance rather than assuming the labelled Wh figure is fully delivered.
Check the inverter and connection method
Some appliances, particularly those with motors, draw a brief starting surge above their normal wattage. Compare both the continuous output rating and surge capability with the appliance specification.
A portable station usually powers appliances plugged directly into it. It does not automatically keep home sockets live when the grid fails. A fixed backup or changeover installation needs suitable equipment and a competent installer.
Never back-feed a house through a plug or improvised cable. Follow UK electrical safety rules and the manufacturer's instructions.
Plan for a longer outage
Test the equipment before an outage, charge it periodically as instructed, store safe lighting and leads with it, and decide which loads take priority. Solar charging is helpful but weather dependent.
Keep essential medical or safety equipment on manufacturer-approved backup arrangements rather than relying on a consumer runtime estimate.
The short version
- Add up watt-hours, not just watts.
- Check continuous power and startup surges.
- A portable station does not automatically power house circuits.
- Plan critical equipment separately.