kW to amps calculator
Convert kilowatts to amps, or amps back to kilowatts, for 230 V single-phase and 400 V three-phase supplies, power factor included. Instant, free, no sign-up.
- Supply–
Guidance only. This gives you the load current. Sizing the protective device and cable from it involves diversity, installation method and voltage drop, run those through the cable size calculator.
The formulas (and a cheat sheet)
To convert kW to amps, multiply the kilowatts by 1,000 to get watts, then divide by the voltage times the power factor. On a UK 230 V single-phase supply at unity power factor that boils down to kW × 4.35 = amps, so 3 kW is 13 A and 9.5 kW is 41.3 A. On a 400 V three-phase supply, divide by √3 × 400 × pf instead, which works out at roughly kW × 1.44 = amps per phase. Sizing an EV charger circuit? The EV charger cable size calculator starts from the same sum.
Written properly: single-phase is I = P ÷ (V × pf) and three-phase is I = P ÷ (√3 × V × pf), where P is in watts and V is the 400 V line voltage. Going the other way, amps to kW, just multiply instead of divide. For volts, amps, ohms and watts in any combination, use the Ohm's law calculator.
Step by step
- Read the kW rating off the nameplate and multiply by 1,000 to get watts. A 9.5 kW shower is 9,500 W.
- Pick the supply: 230 V single-phase or 400 V three-phase.
- Set the power factor: 1 for showers, heaters and anything else that just makes heat, the nameplate figure for motors, usually 0.8 to 0.9.
- Divide the watts by V × pf for single-phase, or by 1.732 × V × pf for three-phase.
- Treat the answer as load current, then think diversity, device rating and cable size before anything gets connected.
Worked examples
A 9.5 kW shower on single-phase: 9500 ÷ 230 = 41.3 A, which is why big showers end up on a 45 A or 50 A device with cable to match. A 22 kW three-phase EV charger: 22,000 ÷ (1.732 × 400) = 31.8 A per phase, call it 32 A on each line. And to show what power factor does: a 3 kW single-phase motor at pf 0.8 pulls 3000 ÷ (230 × 0.8) = 16.3 A, not the 13 A the wattage alone suggests. Skip the pf on motors and your sums flatter the circuit.
kW to amps conversion table
At unity power factor and nominal UK voltages. Three-phase figures are per phase.
| kW | Amps at 230 V single-phase | Amps at 400 V three-phase |
|---|---|---|
| 1 kW | 4.3 A | 1.4 A |
| 2 kW | 8.7 A | 2.9 A |
| 3 kW | 13.0 A | 4.3 A |
| 5 kW | 21.7 A | 7.2 A |
| 7 kW | 30.4 A | 10.1 A |
| 10 kW | 43.5 A | 14.4 A |
| 15 kW | 65.2 A | 21.7 A |
| 20 kW | 87.0 A | 28.9 A |
| 22 kW | 95.7 A | 31.8 A |
Spot how the single-phase column runs out of road: a 100 A domestic cut-out is only about 23 kW flat out, so once a single load creeps into the high teens of kW you're into three-phase territory, or load management if it's an EV charger.
Going the other way, amps to kW
Handy when the clamp meter is doing the talking. Multiply instead of divide: 40 A on a 230 V single phase is 40 × 230 = 9,200 W, call it 9.2 kW. On three-phase, 32 A per phase at unity power factor is 1.732 × 400 × 32 = 22 kW or thereabouts. The calculator above flips direction with one tap, so you can go from a measured current to a load figure for the certificate without touching the formula.
Common UK loads at a glance
| Load | Rating | Supply | Current (approx.) |
|---|---|---|---|
| Kettle | 3 kW | 230 V 1-ph | 13 A |
| Immersion heater | 3 kW | 230 V 1-ph | 13 A |
| Oven (single) | 3.5 kW | 230 V 1-ph | 15.2 A |
| Induction hob | 7.4 kW | 230 V 1-ph | 32.2 A |
| Shower | 8.5 kW | 230 V 1-ph | 37 A |
| Shower | 9.5 kW | 230 V 1-ph | 41.3 A |
| Shower | 10.5 kW | 230 V 1-ph | 45.7 A |
| EV charger | 7.4 kW | 230 V 1-ph | 32 A |
| EV charger | 22 kW | 400 V 3-ph | 32 A / phase |
Ratings assume unity power factor and nominal voltage. Manufacturers rate some showers at 240 V, so the current at 230 V comes out a touch lower than the badge suggests, always check the data plate.
The mistakes that catch people out
- Typing watts into a kW field. If the answer looks a thousand times too big, that's why.
- Using 240 V out of habit. The UK nominal is 230 V, and that's the number your certificates, your tables and this calculator all work from.
- Confusing kW with kWh. kW is power, the rate right now. kWh is energy, power over time, and it's what the bill counts. The conversion to amps only ever uses kW.
- Ignoring power factor on motors and compressors. The real current comes out higher than the wattage suggests, and it's the current the cable feels, not the watts.
What you do with the number
- The current you calculate here is the load current, not the breaker size. Apply diversity where it's allowed, then pick the device.
- Three-phase current is per phase, a balanced 22 kW load pulls 32 A on each of the three lines, not 96 A on one.
- Working in watts or kVA instead? Use the watts to amps calculator or the kVA to amps calculator, same engine, right units.
- Next step: check the run with the voltage drop calculator and size the cable with the cable size calculator.
kW to amps FAQs
How many amps is 1 kW at 230 V?
About 4.35 A at unity power factor. So 3 kW is 13 A (hello, plug tops), and 9.5 kW is 41.3 A.
How do I convert kW to amps on three-phase?
Divide the watts by √3 × 400 × power factor, where √3 is 1.732 and 400 V is the UK line voltage. At unity power factor that's watts ÷ 693, so a 22 kW load draws about 32 A per phase. The √3 is there because the three phases share the load with their voltages 120° apart.
What power factor should I assume?
1.0 for anything that just makes heat: showers, immersions, kettles, ovens. Motors, compressors and transformers typically sit between 0.8 and 0.9. When in doubt, read the nameplate, it beats guessing.
What's the difference between kW and kVA?
kW is real power doing useful work, kVA is the apparent power the supply must deliver. kW = kVA × power factor. For resistive loads they're identical, for motors the kVA is always a bit higher.
What breaker do I need for a 9.5 kW shower?
41.3 A of load means a 45 A device in practice (the next standard size up), with cable sized to suit the route, usually 10 mm² T&E. Check your actual run in the cable size calculator.
What's the difference between kW and kWh?
kW is power, the rate energy is being used right now. kWh is energy, power sustained over time: a 3 kW heater running for two hours uses 6 kWh. Converting to amps only ever involves the kW figure, the kWh figure is what the electricity bill counts.
How many amps does a 7 kW EV charger draw?
About 30 A at 230 V (7000 ÷ 230 = 30.4 A), which is why home chargers sit on a dedicated 32 A circuit. Most "7 kW" units are actually rated 7.4 kW, which is the full 32 A. Factor that circuit in before promising a customer an easy install.
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