What is the formula for converting watts to amps?

For DC circuits: I (A) = P (W) / V (V). For single-phase AC: I = P / (V × PF). For three-phase AC: I = P / (√3 × V × PF), where √3 is approximately 1.732. All three formulas are rearrangements of the power equation P = V × I × PF.

What is power factor and why does it matter?

Power factor (PF) measures how efficiently electrical power is converted into useful work. A PF of 1.0 means all current is doing real work; a PF of 0.8 means 20 % of the current is reactive and produces no useful output. A lower PF means higher current for the same real-power output, so cables and fuses must be sized accordingly. Induction motors, fluorescent ballasts, and switch-mode power supplies typically have PF between 0.7 and 0.95.

How do I convert kilowatts (kW) to amps?

Convert kW to W first by multiplying by 1000, then apply the appropriate formula. For example, a 3 kW single-phase heater at 230 V with PF = 1 draws 3000 / (230 × 1) = 13.04 A. A 5.5 kW three-phase motor at 400 V with PF = 0.9 draws 5500 / (1.732 × 400 × 0.9) = 8.83 A per phase.

What is the difference between VA and watts?

Watts (W) measure real power — the energy actually converted into heat, light, or motion. Volt-amps (VA) measure apparent power — the total current demand on the supply, including any reactive component. For a purely resistive load (PF = 1) they are equal. For motors, capacitors, and inductors, VA is always greater than or equal to W. Fuses, cables, and transformers must be rated for VA, not just W.

How many amps does a 1,000-watt appliance draw?

It depends on the supply voltage and power factor. At 120 V DC or AC with PF = 1: 1000 / 120 = 8.33 A. At 230 V with PF = 1: 1000 / 230 = 4.35 A. At 230 V with PF = 0.9: 1000 / (230 × 0.9) = 4.83 A. Always check the voltage and power factor of your specific circuit.

What three-phase voltage should I use?

Enter the line-to-line (phase-to-phase) voltage, not the phase-to-neutral voltage. In Europe and the UK the standard three-phase supply is 400 V line-to-line (230 V line-to-neutral). In North America it is commonly 208 V or 480 V line-to-line. If your motor nameplate shows 230/400 V, use 400 V for the typical European wye-connected supply.

Watts to Amps Calculator

A watts-to-amps calculator that handles all three common electrical circuit types — DC, single-phase AC, and three-phase AC — with correct power-factor treatment and a worked result display so you can see exactly how the figure was derived.

Why watts and amps are different quantities

Watts measure power — the rate at which energy is consumed or transferred. Amps measure current — the rate of charge flow through a conductor. The relationship between the two depends on the supply voltage and, for AC systems, the power factor of the load. Confusing the two is one of the most common errors when sizing cables, fuses, and circuit breakers; this calculator makes the conversion explicit and shows the formula at each step.

How the calculation works

DC circuits are the simplest case. Ohm’s law and the power equation combine to give:

I = P / V

where I is current in amps, P is power in watts, and V is voltage in volts. This applies to batteries, USB-powered devices, solar panels, and 12 V/24 V vehicle electrical systems.

Single-phase AC circuits introduce a power-factor term because the current and voltage waveforms may be out of phase:

I = P / (V × PF)

Real power (W) is the useful work done; apparent power (VA) is V × I, the total current burden on cables and fuses. When PF = 1 (purely resistive loads such as electric heaters and incandescent bulbs) the two equations are identical.

Three-phase AC circuits carry three conductors, each 120° apart in phase. The standard formula uses the line-to-line voltage and the factor √3 (≈ 1.7321):

I = P / (√3 × V × PF)

This gives the current per phase conductor. Three-phase power is the standard for industrial motors, large HVAC compressors, and modern EV fast-chargers (CCS and CHAdeMO typically deliver 50–350 kW via three-phase AC or DC).

Worked examples

Example 1 — DC solar panel output: A 300 W solar panel operating at 36 V (open-circuit) has no AC power factor. I = 300 / 36 = 8.33 A.

Example 2 — UK electric kettle: A 2,000 W kettle on a 230 V supply. The heating element is purely resistive so PF = 1. I = 2000 / (230 × 1) = 8.70 A. A standard 13 A BS 1363 plug fuse is adequate.

Example 3 — Three-phase industrial motor: A 7.5 kW (7,500 W) motor on a 400 V three-phase supply with PF = 0.87. I = 7500 / (1.732 × 400 × 0.87) = 7500 / 602.7 = 12.44 A per phase. A 16 A Type C MCB is the correct upstream protection.

Example 4 — EV home charger: A 7.4 kW single-phase EV charger at 230 V with PF = 0.99. I = 7400 / (230 × 0.99) = 32.47 A. This requires a dedicated 40 A MCB and 6 mm² cable under most UK wiring regulations.

Apparent power (VA) vs real power (W)

The calculator also shows apparent power in VA for AC modes. This matters because:

Cable current ratings are based on actual current (A), which depends on VA.
Fuse and MCB trip ratings are in amps (derived from VA, not W).
Transformers and UPS systems are rated in VA, not W.
A device with a poor power factor (e.g., PF = 0.6) draws significantly more current than its watt rating alone would suggest, potentially tripping a correctly-sized fuse.

Always size your protective devices and wiring for the apparent current (A) shown in this calculator, not the nameplate wattage alone.