Before You Buy a Power Bank for Your Emergency Kit: 6 Things the Label Won’t Tell You

A “10,000 mAh” power bank does not hold 10,000 mAh for your phone. The number is rated at the cell’s 3.7 volts; your phone charges at 5 volts, so the same energy is about 7,400 mAh before conversion losses, and less after them. Budget on roughly two full charges of a modern phone from a 10,000 mAh bank, not three, and buy by watt-hours (Wh), the figure printed on the label because the FAA requires it.

Ready.gov’s kit list includes a cell phone with chargers and a backup battery, and its power-outage guidance says to plan for a portable charger or power bank. This is the short version of what to know before you buy one for the kit — the things the product page does not say.

1. The capacity is not what you think

The arithmetic, so you can check it yourself: capacity in mAh × cell voltage ÷ 1,000 = watt-hours. A 10,000 mAh bank at 3.7 V holds 37 Wh. Delivered at the 5 V of a USB port, 37 Wh is 7,400 mAh; the boost converter and the phone’s own charging circuit then take their share, so the usable figure lands somewhere in the 6,000-7,000 mAh region. A phone with a 4,000-5,000 mAh battery therefore gets one and a half to two charges, not the “three” the mAh maths suggests. Compare banks by Wh, which is the honest unit, and size the kit on two full phone charges per person per day of outage you plan for.

2. It is empty when you need it unless you recharge it

Lithium cells self-discharge slowly, and the bank’s own electronics draw a little; a bank charged in January and left in the kit is noticeably down by summer. Ready.gov’s tech guidance is to have devices fully charged before severe weather and to keep a backup source on hand — which only works if the backup was charged too. Put the bank on the same rotation as the kit food: top it up on the days you change the clocks, and again at the start of your local storm season. Our annual calendar has the slot.

3. Cold cuts it

Lithium batteries deliver less of their charge when cold and can refuse to charge below freezing. A bank stored in a car through a northern winter, which is exactly where Ready.gov suggests keeping a portable charger, will disappoint in February. Keep the car bank inside your coat or in the cabin, not the trunk, once temperatures drop, and expect the winter kit to need a larger bank than the summer one. Our guide on surviving extreme cold at home covers the rest of the winter kit.

4. The label matters for more than flying

The FAA’s PackSafe rules limit lithium-ion batteries to 100 Wh each in carry-on without airline approval, require power banks to travel in the cabin rather than in checked bags, and require the terminals to be protected from short circuit. Two practical consequences for a kit bank: the Wh rating is on the label of any legitimate product (if it is missing, so is your basis for trusting the mAh claim), and a bank that stays under 100 Wh — roughly 27,000 mAh at 3.7 V — can go anywhere with you, including an evacuation flight. Bigger than that is a small power station, a different product with different rules; see our off-grid power guide.

5. Pass-through and solar are two different questions

A bank that can charge while it is charging a phone (pass-through) is what makes a folding solar panel useful in an outage: panel into bank in the day, bank into phone at night. Many cheap banks cannot do it. The tiny solar cell glued to some “solar power banks” is decoration; a bank of any real capacity needs a separate panel with a proper output, which is what our folding solar charger review compares. The hand-crank radios in our radio buyer’s guide also charge a phone, slowly, and are the fallback when both the bank and the sun are gone.

6. What actually matters on the spec sheet

• Wh, not mAh. See point 1.
• USB-C in and out, and a cable that matches every phone in the family. A kit with a Lightning phone, a USB-C phone and a micro-USB headlamp needs three cables in the bag, or one bank with the right ports.
• A charge indicator you can read in the dark: four LEDs beat a percentage screen that only wakes when you press a button you cannot find.
• A recognised brand and a real safety mark. A power bank is a lithium cell in a plastic box in a bag with your documents; the FAA’s page lists what happens when they overheat. Cheap unbranded cells are where the savings come from.
• Weight. A 20,000 mAh bank is roughly the weight of a can of soup; one per adult in the go-bag is enough, with a small 10,000 mAh in each car.

What to buy

For most families: one 20,000 mAh USB-C power bank per adult in the home kit (about two days of phone use for two people), and a 10,000 mAh bank in each car’s car kit, all under 100 Wh. Check the Wh figure on the listing before the price, and recharge them twice a year. That is the entire decision.