What to look for in an outage power bank: watt-hours over mAh, USB-C PD, airline limits and how to work out how many phone charges you really get.
- Compare banks by watt-hours: Wh = mAh x volts / 1,000.
- Expect a few phone charges from a typical large bank after losses; divide bank Wh by phone Wh and subtract 20 to 30 percent.
- For flying, banks of 100 Wh or less are generally allowed in carry-on; check the airline.
A power bank is the cheapest way to keep a phone alive through an outage. It will not run a fridge or a furnace, but it can keep you connected to alerts, maps, family and a flashlight. The trouble is that packaging numbers are confusing, and the figure in big print is rarely the one that matters.
This guide shows how to read the specs, how to work out the real number of phone charges, and what to look for so the bank is ready when you need it.
Look at watt-hours, not just mAh
Most banks advertise capacity in milliamp-hours (mAh). That number is measured at the battery cell voltage, usually around 3.6 to 3.7 V, not at the 5 V a USB port delivers. Two banks with the same mAh can hold different amounts of energy if their cell voltages differ, so the fairest comparison is watt-hours (Wh).
The formula is simple: Wh = mAh x volts / 1,000. For a 3.7 V cell, a 10,000 mAh bank is about 37 Wh. A 20,000 mAh bank is about 74 Wh. If the label already gives Wh, use it.
How many phone charges will you really get?
Look up your phone's battery in its specs. Many current phones hold somewhere around 12 to 20 Wh. Divide the bank's Wh by the phone's Wh, then take off roughly 20 to 30 percent for conversion losses, heat and cable losses.
- Example: a 74 Wh bank, a 15 Wh phone. 74 / 15 is about 4.9. Subtract losses and you get roughly 3 to 4 full charges.
- Tablets hold much more, often 25 to 45 Wh, so one charge uses a big share of a small bank.
- Charging from 20 percent to full uses less than charging from empty, so real-world charges often feel better than the maths.
To size for a household, count each person's phone and add a margin for a radio or headlamp. A family of four that wants two charges each will need several hundred Wh in total, which may mean more than one bank or a larger unit. For bigger loads see the power stations hub.
Features worth having
- USB-C Power Delivery (PD): A standard that lets the bank and the device agree on a faster, safe charging speed. Useful for phones, tablets and some small laptops. Check the bank's maximum output and your device's needs.
- Pass-through charging: Lets you charge the bank while it charges your phone. Handy but not always supported.
- Multiple ports: Two or more outputs mean you can charge two devices at once. Remember they share the total output.
- Input speed: A bank that takes a long time to recharge is a problem if you only have a few hours of power at the neighbour's or in the car.
- Built-in safety: Look for protection against overcharge, short circuit and overheating, and for recognized safety certification marks that apply where you live.
- Low-current mode: Helpful for small gadgets such as earbuds or a headlamp that might otherwise be ignored by the bank.
- Built-in light: A nice extra, but do not rely on it as your only light. See headlamps and lanterns.
Travel and airline rules
Power banks contain lithium cells, so they must go in carry-on baggage, not checked bags. In general, banks of 100 Wh or less are allowed in carry-on. Banks above 100 Wh and up to 160 Wh usually need airline approval, and anything larger is generally not allowed. Rules can change and airlines vary, so check yours before flying. If you plan to keep a bank in your go-bag for an evacuation, a size under 100 Wh avoids travel problems.
Keep it ready
- Charge it fully when you buy it, then top it up every two to three months. Lithium batteries self-discharge slowly.
- Store it at room temperature, not in a hot car or a freezing garage.
- Pack the right short cables, including a spare for each device type in the house.
- Do not use a bank that is swollen, damaged, very hot or leaking. Stop using it and take it to a proper battery recycling drop-off.
- Put one in the 72-hour kit and one in your everyday bag.
When a power bank is not enough
If you also need to run a CPAP, a small fan, a router or other devices, you will want a larger battery. A power bank is a short-term phone solution. Use the outage planner to find your own needs.
Quick answers
Is 20,000 mAh enough for a power outage?
It is about 74 Wh at 3.7 V, which gives several phone charges but not much more. It will not run appliances.
Can I put a power bank in checked luggage?
No. Lithium batteries should travel in carry-on bags. Check your airline for limits.