Watts to amps calculator for UK supplies

How to convert watts to amps

On a single-phase supply, amps = watts ÷ volts. In the UK that means dividing by 230. A 3,000W kettle draws 3,000 ÷ 230 = 13.04A. This is the example the calculator opens with: watts to amps, single-phase, 230V, power factor 1.

Why 230 and not 240? The Electricity Safety, Quality and Continuity Regulations 2002 set the declared low voltage supply at 230 volts between phase and neutral, allowed to run 10% above or 6% below. So your meter might read 240V on the day, but 230V is the figure to calculate with. Change it in the calculator if you have a reason to.

Going the other way, watts = amps × volts. A 32A circuit at 230V is 32 × 230 = 7,360W, or 7.36kW.

Where the power factor comes in

Power factor is the ratio of the real power a load uses, in kW, to the apparent power it draws, in kVA. That is Fluke's definition, and apparent power is simply volts × amps. A load with a power factor below 1 draws more current than its watts alone suggest.

So the full single-phase formula is amps = watts ÷ (volts × power factor). For heating loads such as kettles, showers, immersion heaters and fan heaters, leave it at 1 and the formula reduces to watts ÷ volts. For motors, compressors and equipment with a power supply inside, the power factor is lower, and the figure on the rating plate is the one to use.

Better still, if the rating plate gives the current, use that. It is the maker's own figure, and it beats any conversion.

Three-phase: kW to amps at 400V

For a balanced three-phase load, amps = watts ÷ (√3 × volts × power factor), where the volts are measured line to line. √3 is about 1.732. The formula, P = √3 × V × I × PF, is the one set out by Electricity Forum. The answer is the current in each line, not the total.

In the UK the line-to-line voltage is 400V. The government's chargepoint specification uses exactly these two figures: 230V single-phase, 400V three-phase. They are the same supply seen two ways: 230 × √3 = 398, close to 400.

• 11kW at 400V, power factor 1: 11,000 ÷ (1.732 × 400) = 15.88A per line.
• The same 11kW load at power factor 0.85: 11,000 ÷ (1.732 × 400 × 0.85) = 18.68A per line. The lower power factor adds nearly 3A.
• 22kW at 400V, power factor 1: 22,000 ÷ (1.732 × 400) = 31.75A per line.
• Back the other way, 32A per line at 400V: 1.732 × 400 × 32 = 22,170W, or 22.17kW.
• For comparison, 7kW on single-phase 230V: 7,000 ÷ 230 = 30.43A. Three-phase carries about three times the power for a similar current per line.

Amps for common UK appliances, and what fits on a plug

These ratings are the ranges National Energy Action gives for household appliances, converted to amps at 230V with a power factor of 1. Models vary, so the rating plate on the actual appliance is always the better figure. Washing machines and tumble dryers do not draw their full rating for the whole cycle.

A 13A plug at 230V is good for 13 × 230 = 2,990W. National Energy Action's kettle and fan heater ratings top out at 3,000W, right at that limit. Greater Manchester Fire and Rescue Service gives the rule of thumb: a 3A fuse for appliances up to about 700W, and a 13A fuse for those between about 700W and 3,000W, the maximum rating of a wall socket.

Anything bigger, such as an electric shower, a cooker or an EV charger, needs its own circuit from the consumer unit. Converting its kW to amps is the first step in sizing that circuit. The cable size calculator takes the next step and picks a twin and earth size for the way the cable is installed.

• Electric shower, 7,000 to 10,500W: 30.43 to 45.65A.
• Immersion heater, 3,000W: 13.04A.
• Kettle, 2,500 to 3,000W: 10.87 to 13.04A.
• Fan heater, 1,000 to 3,000W: 4.35 to 13.04A.
• Grill or oven, 2,000 to 2,400W: 8.70 to 10.43A.
• Washing machine, 1,000 to 2,400W: 4.35 to 10.43A.
• Tumble dryer, 1,500 to 2,500W: 6.52 to 10.87A.
• Microwave, 700 to 1,400W: 3.04 to 6.09A.
• Home EV charger, 3.5 to 7kW single-phase, the standard AC band in the government specification: 15.22 to 30.43A.

Watts, kWh and what it costs to run

Amps tell you whether the circuit can carry the load. Kilowatt hours tell you what it costs. A kWh is one kilowatt used for one hour, and it is the unit on the electricity bill.

National Energy Action gives the sum: running cost in pence per hour = power rating in watts × unit rate in pence per kWh ÷ 1,000. Your unit rate is on your bill.

So a 3,000W immersion heater at a unit rate of 25p per kWh costs 3,000 × 25 ÷ 1,000 = 75p an hour, while it draws 13.04A. The 25p is only an example: put in your own rate. A thermostat switches the element off once the water is hot, so the real cost depends on how long it actually runs.

What the conversion will not tell you

It gives the current a load draws, nothing more. It does not choose a cable, a breaker or a fuse, and it knows nothing about the extra current some equipment, such as a motor, draws for a moment as it starts. For a whole house or a workshop, adding up every appliance's amps can also overstate the load, since they are rarely all on at once.

It is a guide only. Any new circuit, and any wiring work beyond plugging in an appliance, needs designing and certifying by a qualified electrician under BS 7671. New circuits in England are also notifiable work under Approved Document P.

Common questions