Anker PowerCore USB-C power bank

What it is good at

• Charging a phone through a long day away from mains power
• Topping up a small laptop over a high-output USB-C port
• Holding its charge for months when left in a bag

Check before you buy

• The watt-hour figure on the label against the airline carry-on limit
• Output per port: a 20,000 mAh pack rarely charges a large laptop at full speed
• Whether it recharges over USB-C, which is far quicker than micro-USB

Who it is not for

Anyone who only needs one extra phone charge, for whom a small 10,000 mAh pack is lighter and enough.

Capacity is quoted in milliamp-hours at the cell voltage, so a big number does not mean a big charge. Watt-hours, which combine capacity and voltage, are the honest figure, and they are also the number an airline checks at the gate.

Output matters as much as capacity for a laptop. Most packs will run a phone at full speed, while a laptop needs a high-wattage USB-C port and may still charge only while idle, and a pack left on a shelf slowly loses charge over months.