How long will a 1500Wh battery last during a power cut? The honest answer depends mainly on what you plug into it. A 1500Wh portable power station might run a router and a few lights for a long time, but a kettle, heater or other high-power appliance can use much of its stored energy quickly.
The calculation is straightforward, provided you use the appliance’s actual running wattage and allow for conversion losses. The difficult part is estimating a realistic household load, particularly where appliances cycle on and off or use a high starting surge.
What does 1500Wh mean?
Wh means watt-hours, a measure of stored energy. In simple terms, a 1500Wh battery could theoretically provide:
- 1500 watts for one hour;
- 750 watts for two hours; or
- 150 watts for ten hours.
Those figures are a useful starting point, not a promise of actual runtime. A battery’s quoted capacity is normally its nominal storage figure. You may not be able to use every watt-hour because of the battery management system, a reserve limit, inverter losses and the manufacturer’s operating limits.
If you are powering mains appliances from a portable power station, electricity also passes through an inverter. Its efficiency varies with load and model. The result is that the energy available at the AC sockets is lower than the battery’s nominal capacity.
How long will a 1500Wh battery last?
For a rough estimate, use:
Runtime in hours = usable battery energy in Wh ÷ appliance load in watts
For a more realistic estimate, start with the nominal capacity and apply an allowance for usable capacity and inverter efficiency:
Estimated runtime = 1500Wh × usable-capacity factor × inverter-efficiency factor ÷ running load
For illustration only, suppose 85% of the quoted capacity is available for use and the inverter delivers 90% of the DC energy to the AC output. That gives approximately 1148Wh of usable AC energy:
1500 × 0.85 × 0.90 ≈ 1148Wh
With those assumptions, a continuous 100W load would run for about 11.5 hours. A continuous 500W load would run for about 2.3 hours. A 1000W load would run for roughly 1.1 hours.
These are planning estimates. Check the power station’s manual for its usable capacity, inverter rating, low-battery cut-off and any guidance about sustained loads.
Example runtimes for common household loads
The table below uses approximately 1148Wh of usable AC energy. The appliance wattages are examples rather than fixed UK values: check the data plate, power meter or manufacturer’s specification for your own equipment.
| Example load | Illustrative running load | Approximate runtime |
|---|---|---|
| Wi-Fi router | 10W | About 115 hours |
| Router plus LED lights | 30W | About 38 hours |
| Laptop | 60W | About 19 hours |
| Router, lights and laptop | 100W | About 11.5 hours |
| Television | 100W | About 11.5 hours |
| Television plus router and lights | 130W | About 8.8 hours |
| Small fridge while its compressor is running | 150W | About 7.7 hours |
| Portable heater | 1500W | Less than one hour |
The fridge example needs particular care. A fridge does not normally draw its running wattage continuously: the compressor switches on and off in response to temperature. Its average consumption may therefore be lower than the instantaneous running figure, but the compressor can demand a higher starting surge. A power station must support both the continuous load and that brief start-up demand.
The same caution applies to pumps, freezers, fans with motors and some power tools. Look for both running and starting information in the appliance or power-station documentation. If the manufacturer gives only a vague wattage range, use a plug-in power meter where appropriate rather than treating the lowest figure as representative.
Household combinations: the load adds up
Runtime is determined by the total load, not by the number of appliances. For example, a modest emergency setup might include:
- router: 10W;
- two LED lamps: 20W in total;
- laptop charger: 60W; and
- television: 100W.
That is an illustrative combined load of 190W. Using the same 1148Wh planning figure gives approximately six hours, assuming those devices draw their stated power continuously. In real use, the laptop may finish charging, the television may be switched off, and the lamps may not be used all night. A measured or time-weighted average load could therefore produce a different result.
By contrast, adding a 3kW kettle or a 2kW fan heater is likely to dominate the calculation. Even if used for only a short period, a high-power appliance can consume more energy than several hours of lighting, networking and laptop use. It may also exceed the power station’s continuous AC output rating.
Why the advertised runtime can be different
Usable capacity is not the same as nominal capacity
Manufacturers quote battery capacity under specified conditions. The energy available at the sockets can be reduced by reserve capacity, temperature, battery age, discharge limits and the conversion path used. Some products also quote separate figures for DC outputs and AC outputs.
Small loads can have their own overhead
An inverter may consume some power even when the connected appliance is using very little. This matters more with a 5W or 10W load than with a 500W load. If the power station has an energy-saving mode, check how it behaves before relying on it for a small always-on device.
Appliances do not all draw power continuously
Fridges, freezers, boilers and some heating or cooling equipment cycle. Chargers also reduce their draw as a battery fills. A calculation based on a constant maximum wattage can be conservative, while one based on an appliance’s lowest figure can be misleading. The useful number for runtime is the average load over the period you care about.
Temperature and battery condition matter
Very cold or hot conditions can affect battery performance and the equipment’s permitted operating range. Capacity can also decline with age and use. Follow the manufacturer’s storage, charging and temperature guidance, particularly if the power station is kept in an unheated shed or loft.
Check the power station before connecting appliances
Capacity is only one part of sizing. Check the specification for:
- continuous AC output: the power it can supply for sustained operation;
- surge or peak output: the short-term capability for starting compatible motor loads;
- output waveform and socket limits: relevant to some equipment and adapters;
- DC and USB output limits: these may be separate from the AC inverter rating; and
- charging options and recharge time: useful if the outage lasts beyond one discharge cycle.
Do not assume that a 1500Wh battery can run a 1500W appliance for exactly an hour, or that it can run any appliance whose wattage is below 1500W. The output rating, surge demand and conversion losses still apply.
For a more tailored estimate, list the appliances you expect to use, their running watts, how many hours each will operate, and any known starting-surge requirement. The UK Backup Power Calculator can help turn that appliance list into a runtime and sizing estimate. Use the figures from your own equipment rather than generic examples wherever possible.
What can a 1500Wh battery realistically cover?
A 1500Wh unit is often more useful for prioritised essentials than for keeping an entire home operating normally. A sensible outage plan might prioritise communications, lighting, laptops and selected refrigeration. High-demand heating and cooking appliances can use the available energy rapidly and may not be compatible with the inverter at all.
It is also worth deciding whether you need continuous power or occasional use. Running a router continuously has a different energy requirement from charging a phone several times a day. Similarly, using a kettle briefly may be practical if the power station supports the load, even though leaving a heater on would not be.
Keep combustion generators separate from indoor battery equipment. A petrol, diesel or gas generator must not be operated indoors, in a garage, basement or another enclosed or poorly ventilated area because of carbon-monoxide risk. The Health and Safety Executive’s generator guidance explains key safety considerations. Fixed connections to household wiring require appropriate professional advice; do not attempt backfeeding or consumer-unit modifications.
Frequently Asked Questions
Can a 1500Wh battery run a fridge during a power cut?
It may, but the answer depends on the fridge’s average energy use, compressor starting surge and the power station’s AC output and surge ratings. A simple calculation using the compressor’s running wattage can overstate or understate the real result. Check the appliance and power-station specifications, and allow for inverter losses.
How long will a 1500Wh battery run a 100W appliance?
Using the illustrative assumption of about 1148Wh available at the AC output, a continuous 100W appliance would run for approximately 11.5 hours. A different usable-capacity figure, inverter efficiency, battery condition or appliance duty cycle will change that estimate.
Can a 1500Wh battery run a kettle or heater?
Only if its inverter is rated for the appliance’s power and any starting demand. A high-power kettle or heater can drain the battery quickly, even during a short use, so check the exact wattage and the power station’s continuous output limit before connecting it.
To plan for your next outage, read the wattage labels on the appliances you actually want to keep running and enter those figures into the UK Backup Power Calculator. That will give you a more useful estimate than relying on the 1500Wh label alone.