Charger watts: what the numbers on the label mean

Total watts are shared, not multiplied

The headline figure is what the charger can deliver in total. Plug in a second device and that budget is divided, which is why a laptop that charges quickly alone can crawl once a phone joins it. The power table printed on the case shows the split.

For most people one high-output port is worth more than several small ones, because the laptop is the device that actually needs the watts.

• USB-C Power Delivery negotiates a voltage and current pair, so both ends must support the same profile.
• A charger rated above what the laptop asks for is fine; it will be asked for less.
• Cables have ratings too, and a thin one can cap the speed of a good charger.

Overnight charging behaves differently

Wireless pads and slow overnight charging are about convenience rather than speed, and a phone left on a pad for seven hours does not need a fast supply. The loss in the coil turns into a little heat, which is normal and the reason a pad should sit somewhere with air around it.

For batteries, staying between roughly a fifth and four fifths of capacity on ordinary days is kinder than running to empty, and most phones now manage that automatically overnight.

Where each device belongs

A desk charger stays plugged in and handles everything at once; a power bank travels and only has to refill a phone or top up a small laptop. Buying one device for both jobs tends to mean a charger too heavy to carry and a pack too weak for the desk.

• Desk: a multi-port supply with one port that can carry the laptop.
• Bag: a pack with a high-output USB-C port and a stated watt-hour figure.
• At the laptop: a hub for the peripherals, powered from the same supply.