Start with what must stay charged
- List devices
Write down the phones and other USB devices you would actually need during a power cut. Check each device or its documentation for battery energy in Wh, if available.
- Count charges or top-ups
Decide how many full charges or partial top-ups each device needs before mains power is likely to return. Saving phone battery and charging it before a forecast interruption may reduce what you need to buy.
- Check energy and output
Add the device energy required, leave room for conversion losses, then check that the bank's per-port output and cable support each device. A laptop may need a compatible USB-C Power Delivery output that a phone-only bank does not provide.
Read the three ratings separately
- mAh
- Stored electric charge at a stated battery voltage. It is not a count of phone charges.
- Wh
- Stored energy. This is the clearer starting point for comparing battery and device energy.
- W output
- The rate a port can deliver power. Enough Wh does not guarantee that a port can charge a laptop or another demanding device.
A simple example, with the limits visible
The conversion is Wh = mAh x nominal battery voltage / 1,000. Anker gives the example of 10,000mAh at 3.7V, which is about 37Wh stored. Use the voltage printed for the particular bank rather than assuming every product uses 3.7V.
Suppose a phone battery is rated at 15Wh and you want two full charges. That is 30Wh reaching the phone battery in this hypothetical example. A 37Wh bank might look sufficient on paper, but charging and voltage conversion use some energy. It cannot be promised to provide two complete charges. The phone's actual battery rating, its use while charging and the bank's delivered energy all matter.
If the maker provides an output-energy figure for the relevant port, use that for planning. Otherwise, treat a division of advertised bank Wh by phone Wh as an upper bound, not an expected result. A larger capacity adds weight and may not solve a weak output port.
Check fit before buying
- Match the charging connection
Check the device's connector, charging standard and required wattage against the bank's per-port specification. USB-C shape alone does not tell you the power supported by the whole bank, cable and device combination.
- Consider carrying and recharging
Choose a size you can store accessibly and keep charged. Check how the bank itself recharges and whether a suitable charger and cable are already at home.
- Try your own setup
Before relying on it, charge the bank and check that it charges each intended device with your actual cables. This is a household compatibility check, not a published performance test.
Charge and store it safely
London Fire Brigade advises using a suitable charger, keeping batteries away from heat and liquids, and not covering chargers. Stop using a bank that is damaged, swollen or too hot to handle. Choose a reputable seller, register the product for recall notices where possible and use a proper battery recycling route at end of life.
What a power bank cannot guarantee
Usable energy varies with the device, cable, output mode, temperature, battery condition and whether the phone is being used while charging. A charged phone also needs a working mobile network for calls or data. If someone depends on a medical or care device, ask its provider and care team for a specific continuity plan rather than using this guide to size its backup.
Products that may help
Specific options, with the limitations visible.
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Anker Nano Power Bank 10,000mAh with built-in USB-C cable
Anker A1259, 30W, black
A compact USB-C power bank for keeping a compatible phone charged during a short power cut. Check the device connector and charging requirements first.
- Useful for
- A household needing a small, rechargeable reserve for a USB-C phone or another compatible low-power device.
- Probably not for
- Anyone needing to run a fridge, mains appliance, or laptop that needs more than 30W.
- Built-in USB-C cable reduces the chance of forgetting a cable.
- Separate USB-C and USB-A ports add connection options.
- Compact size for a household grab bag.
- 10,000mAh is the rated cell capacity, not the energy delivered to a phone.
- Maximum output is 30W on one USB-C connection; combined output is lower.
- No AC socket or direct fridge supply.
Check the device charging input and use a sound cable and suitable charger. Do not assume a USB power bank can power a router or medical device.