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Free calculator

Use this calculator to

  • Convert nameplate watts into the running current at 230 V single phase
  • Size plug fuses, MCBs and cable runs from a known load in watts
  • Allow for power factor on motor and HVAC loads with the AC tab
  • Estimate current from watts and resistance when the voltage is not stated

Watts to Amps Calculator

Convert power (W) to current (A).

W
V
Result
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W
V
PF
Result
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W
Ω
Result
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Formulas

  • A = W / V
  • A = W / (V × PF) (AC)
  • A = √(W / R)

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Every time a business adds new machinery, an EV charger, a heat pump or a process oven, the first electrical question is the same: what current will it draw, and is the existing cable and breaker rated to carry it. Converting nameplate watts into amps at the right voltage and power factor is the starting point. A 22kW three-phase EV charger pulls around 32A per phase at 400V, which means a 32A breaker is the minimum, with cable sized for the run length to keep voltage drop inside the 5% limit set out in BS 7671.

Mid-market businesses adding capacity at multiple sites need consistent answers across the portfolio. A 50kW process load installed at six bakeries will draw the same current at each site, but the cable run, the ambient temperature and the grouping with other circuits will all be different. The watts to amps conversion gives the consistent reference point. From there, the local design engineer applies the derating factors and signs off compliance against the regulations, with the documentation ready for the next periodic inspection.

Purely Energy helps facilities and operations teams plan capacity additions across multi-site portfolios. We sit between the equipment specification and the supply contract, making sure that when a site adds 100kW of new load, the MIC reflects it, the supply transformer can deliver it and the tariff shape captures the new consumption pattern. We do that work across 30+ panel suppliers, so the procurement decision is not constrained by who happens to have account managed the client last year.

Common questions

How do I convert watts to amps on a 230 V single-phase supply?

Divide the power by the voltage and the power factor: A = W / (V x PF). For resistive loads such as heaters and kettles the power factor is 1, so a 3 kW load on 230 V draws 3000 / 230, about 13 A. For motors, compressors and electronic loads, divide by the power factor as well, which always increases the current. The AC tab on this calculator handles that for you.

What power factor should I use?

Resistive heating and incandescent lighting sit at effectively 1.0. Modern IT equipment and LED drivers with power factor correction are typically 0.9 to 0.95. Induction motors run around 0.8 to 0.9 at full load and drop noticeably when lightly loaded. If you have no datasheet figure, 0.9 is a reasonable starting assumption for mixed commercial loads. Remember the rule of thumb: the lower the power factor, the more amps a circuit carries for the same useful watts.

How do I convert watts to amps on a 400 V three-phase supply?

For a balanced three-phase load, the line current is A = W / (1.732 x VL x PF), where VL is the 400 V line-to-line voltage and 1.732 is the square root of 3. A 30 kW load at power factor 0.9 draws 30000 / (1.732 x 400 x 0.9), about 48 A per line. This page's tool covers single-phase circuits; use the formula above, or the dedicated three-phase calculators, for 400 V supplies.

How many amps does a 3 kW appliance draw in the UK?

At the nominal 230 V mains voltage, a 3 kW resistive appliance draws 3000 / 230, which is 13.04 A. That is why 3 kW is the practical ceiling for anything on a standard BS 1363 plug, whose largest fuse is 13 A. Loads above that, such as electric showers, commercial ovens and EV chargers, are hard-wired on dedicated circuits sized for the running current plus an allowance for the circuit's protective device.

Can I work out amps from watts and resistance alone?

Yes. From P = I squared x R, the current is I = the square root of W / R. The third tab on this calculator uses exactly that formula. It is useful when you have measured a load's resistance with a multimeter but do not know the supply voltage, for example when checking a heating element on the bench. For a 2 kW element of 26.5 ohms, the square root of 2000 / 26.5 gives about 8.7 A.

Related calculators

The conversions that procurement and engineering use alongside this one.

Watts to Amps Calculator | Purely Energy