Guide · 5 min read
Amps, volts and watts.
One equation reads every charger label ever printed: watts = volts × amps. Everything confusing about charger specifications — the 2.4A brick that is only 12W, the 65W charger that is really 20V at 3.25A — falls out of that multiplication.
The three quantities
Volts is electrical pressure — how hard the supply pushes. Amps is flow — how much current actually moves. Watts is the product, the rate of energy delivery, and it is the only one of the three that describes charging speed on its own. A charger label gives you the first two and expects you to do the third.
| Label on the brick | Multiplication | Real output |
|---|---|---|
| 5V ⎓ 2.4A (USB-A) | 5 × 2.4 | 12W |
| 5.2V ⎓ 2.4A (Apple 12W) | 5.2 × 2.4 | 12.5W, sold as 12W |
| 9V ⎓ 2.22A (Apple 20W) | 9 × 2.22 | 19.98W, sold as 20W |
| 9V ⎓ 2.77A (Samsung 25W) | 9 × 2.77 | 24.9W |
| 20V ⎓ 3.25A | 20 × 3.25 | 65W |
| 20V ⎓ 5A | 20 × 5 | 100W — the PD SPR ceiling |
| 28V ⎓ 5A | 28 × 5 | 140W, PD 3.1 only |
The ⎓ symbol means direct current; a ~ means alternating current. When a label lists several output lines, the adapter's advertised wattage is the largest product among them, and that is the rail your device will pick if it can use it.
Why "2.4A" is a much weaker claim than it looks
A USB-A port is fixed at 5V. It has no mechanism to raise voltage without the legacy Quick Charge handshake, so "2.4A fast charging" on a USB-A brick means 5 × 2.4 = 12W, full stop — roughly half of what a 20W USB-C port does and a fifth of a 65W one. This is why every genuinely fast charger raises voltage instead of current: current is limited by what the conductors can survive, while voltage is limited only by the negotiated contract. It is the same reason USB-C PD climbs 5V → 9V → 15V → 20V → 28V → 48V, and why 240W is still only 5A. Comparison: USB-A vs USB-C.
The input line, and watt-hours
The small print above the output usually reads something like Input: 100–240V ~ 50/60Hz. That range is the whole travel answer: the adapter accepts any mains voltage in the world, so you need a plug shape adapter, never a voltage converter. Converters exist for heating elements and motors, not for switch-mode power supplies. Detail in converters vs adapters and the travel hub.
Power banks are rated in milliamp-hours, which is a capacity at an unstated voltage and therefore not directly comparable. Convert it: watt-hours = mAh × cell voltage ÷ 1000. A 20,000mAh bank built on 3.7V cells is 74Wh, which matters because airlines cap spare lithium batteries at 100Wh without approval. That, not the mAh number, is the figure to check before flying — see airline rules for chargers. The same conversion tells you how many phone charges you get: a 74Wh bank against a 17Wh phone battery is about three full charges after conversion losses.
FAQ
Does more amps mean faster charging?
Only at the same voltage. 5V at 3A is 15W; 9V at 2A is 18W despite fewer amps. Compare watts, never amps alone.
What does the tilde and the ⎓ symbol mean on a charger?
The tilde marks alternating current, used for the mains input line. The ⎓ symbol marks direct current, used for the output your device receives.
How do I convert mAh to watt-hours?
Multiply mAh by the cell voltage, usually 3.7V, then divide by 1000. A 10,000mAh bank is about 37Wh.
Why does my 65W charger only deliver 45W?
Either another port is sharing the budget, or the cable is limiting current to 3A, or the device simply never requests more than 45W.