Power bank and power station runtime calculator
Short answer: hours = capacity in Wh × efficiency ÷ watts. Use about 0.85 for a power station's mains sockets and about 0.82 for a USB power bank. A 500 Wh power station runs a 60 W electric blanket for about 7 hours.
Efficiency figures from Jackery and Anker (manufacturers' estimates), read 1 November 2026. No official UK figure exists.
How the maths works
- Stored energy. Power stations show watt-hours (Wh). Power banks show mAh at the cell voltage, so Wh = mAh × 3.7 ÷ 1,000. A 20,000 mAh bank holds about 74 Wh.
- Usable energy. Some energy is lost as heat. Anker gives about 82% for its power banks; Jackery uses 0.85 for its power stations. We use those as the starting values.
- Runtime. Usable Wh ÷ total watts = hours. With the hours a day you set, usable Wh ÷ Wh a day = days.
Worked examples: 1,000 Wh power station, fridge-freezer at 30 W average: 1,000 × 0.85 ÷ 30 ≈ 28 hours. 20,000 mAh power bank, LED lantern at 5 W: 74 × 0.82 ÷ 5 ≈ 12 hours.
Staying safe
- Never run a petrol or diesel generator indoors or in a garage: it gives off carbon monoxide, a poisonous gas you can't see or smell.
- For powered medical equipment, ask your supplier and join the Priority Services Register. Don't rely on a battery alone.
- Keep batteries charged before winter storms, and recharge after each use.
More: do you need a power station for power cuts? · power banks in a power cut · power cuts near me (live) · compensation calculator
Frequently asked questions
How do I work out how long a power station will last?
Multiply its capacity in watt-hours (Wh) by about 0.85 to allow for losses in the mains socket's inverter, then divide by the watts your devices use. For example, 500 Wh × 0.85 ÷ 60 W (an electric blanket) is about 7 hours. Jackery publishes the same 0.85 rule of thumb.
How many phone charges will a 10,000 mAh power bank give?
A 10,000 mAh power bank stores about 37 Wh (10,000 × 3.7 V ÷ 1,000). Anker says about 82% of that reaches your device, so roughly 30 Wh, which is about 2 full charges of a phone with a 15 Wh battery.
How long will a power station run a fridge-freezer?
Find the kWh a year on its energy label and divide by 8.76 to get its average watts: 263 kWh a year is about 30 W. A 1,000 Wh power station would then run it for about 28 hours (1,000 × 0.85 ÷ 30). Fridges draw several times more for a moment when the motor starts, so check the unit's surge rating. Keeping the door shut also helps the food stay cold.
Can I run medical equipment on a power bank or power station?
Ask your equipment supplier how long it will run on battery and what they recommend, and join the Priority Services Register with your network operator. Don't rely on a power station alone.
Should I use a generator in a power cut?
Never run a petrol or diesel generator indoors, in a garage or near windows: it gives off carbon monoxide, a poisonous gas you can't see or smell. A battery power station gives off no fumes, so it can be used indoors.
Why is the efficiency not 100%?
Some energy is lost as heat when the battery's low voltage is raised to 5 V for USB or 230 V for a mains socket. Power banks lose more than the label suggests because mAh is quoted at the 3.7 V cell voltage.
Sources
- Anker: Why is the rated capacity marked on a power bank lower than expected? (about 82% conversion efficiency), read 1 November 2026
- Jackery UK: What is a watt-hour (working time = capacity Wh × 0.85 ÷ watts), read 1 November 2026
- Ofgem: Join your supplier's Priority Services Register, read 28 October 2026
- GOV.UK Be Ready: Power cuts (keep a charged power bank), read 28 October 2026
We don't sell or link to power banks or power stations on this page. The efficiency figures come from two manufacturers because no official UK figure exists.
Civilian Survival Handbook is an independent UK site. We are not affiliated with or endorsed by Ofgem, any network operator, the Met Office, the Environment Agency or the UK Government.