The formulas
- DC: watts = volts × amps.
- Single-phase AC: watts = volts × amps × power factor.
- Three-phase AC: watts = √3 × line volts × amps × power factor.
The power factor allows for loads where current and voltage aren't in step, such as motors and some power supplies. Heaters, kettles and filament lamps have a power factor of 1. A 2 kW kettle on 230 V draws about 8.7 A.
Plug fuses in the UK
UK plugs take 3 A, 5 A or 13 A fuses. A 3 A fuse suits appliances up to about 700 W, and 13 A anything up to 3 kW. Use the smallest fuse that doesn't blow in normal use, so it protects the appliance's cable.
For the radio shack
A 100 W transceiver draws about 20 A at 13.8 V, around 280 W from the supply, and a linear amplifier much more. The DC voltage drop calculator sizes the cables, and the battery and solar calculator plans an off-grid station.
Questions
How many amps is 2000 W at 230 V?
About 8.7 A.
How many amps is 1000 W at 12 V?
About 83 A, which is why 12 V inverters need very thick cables.
How do I convert watts to amps?
Divide the watts by the volts, and by the power factor for AC. For three-phase, also divide by √3.
Which fuse should a 1 kW appliance have?
A 5 A fuse in the UK. Use 3 A up to about 700 W and 13 A above about 1.1 kW.