How many times will a 40,000 mAh power bank charge a laptop over USB-C?
About 1.8 full charges of a typical laptop battery (55 Wh), from 1.6 to 2.1 depending on the bank.
A laptop that charges over USB-C can work from a power bank for most of a day. A 40,000 mAh bank holds about 148 Wh at its 3.7 V cells, of which about 75% typically reaches the USB port, less what the device loses charging its own battery.
The numbers
| Device | Worst case | Typical | Best case |
|---|---|---|---|
| Small laptop (40 Wh battery) | 2.2 | 2.5 | 2.8 |
| Typical laptop (55 Wh battery) | 1.6 | 1.8 | 2.1 |
| Large laptop (75 Wh battery) | 1.2 | 1.3 | 1.5 |
The bank needs a USB-C Power Delivery port rated for at least 45 W, or the laptop charges slowly or not at all. Laptops that use a barrel-plug charger cannot charge from a power bank.
Other sizes
| Power bank | Energy | Typical |
|---|---|---|
| 10,000 mAh | 37 Wh | 0.5 full charges |
| 20,000 mAh | 74 Wh | 0.9 full charges |
| 26,800 mAh | 99 Wh | 1.2 full charges |
| 40,000 mAh | 148 Wh | 1.8 full charges |
Where to buy
A 40,000 mAh power bank
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When a power bank is not enough
A power station runs the same job from a mains socket, for far longer, and runs the things a power bank cannot: the fridge, the boiler's electrics, a mains electric blanket. For example, the Anker SOLIX C300 runs a laptop over USB-C for about 3 h 44 min.
If this is for a power cut
For phones and tablets alone, a power bank is the cheaper answer. A power station earns its place when you also need the router or the fridge.
Power bank, 20,000 mAh. Keeps a phone going for days if you use it sparingly.
Look for: Capacity in watt-hours as well as mAh (about 74 Wh for 20,000 mAh), USB-C in and out, and keep it charged.
Plug-in power failure lights. They live in a socket as a night light, charge themselves, and switch on the moment the mains goes off, so you are never hunting for a torch in the dark.
Look for: A built-in battery you can pull out and carry as a torch, and a UK three-pin plug if you are in the UK.
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More: the power cut essentials list (US edition).
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