Watts to Amps Calculator

The formulas

Power, current and voltage are linked by a simple relationship. In a DC circuit, such as a 12 V battery system, power equals current times voltage. In AC circuits the power factor (PF) also matters: it is 1 for purely resistive loads such as heaters, kettles and incandescent bulbs, and lower for motors and some electronics. In a three-phase circuit, with the line-to-line voltage, the current per line is lower by a factor of √3 (about 1.732).

DC: I = P ÷ V and P = I × V
AC single-phase: I = P ÷ (V × PF)
AC three-phase: I = P ÷ (√3 × V × PF)

Worked examples

• A 1,500 W heater on a 120 V circuit draws 1,500 ÷ 120 = 12.5 A.
• The same heater on a 230 V supply, as in the UK, Europe or Australia, draws only 6.52 A. Higher voltage means less current for the same power.
• A 20 A circuit at 240 V delivers up to 20 × 240 = 4,800 W.
• A 10 kW three-phase motor load at 400 V with a power factor of 0.85 draws 10,000 ÷ (1.732 × 400 × 0.85) = 16.98 A per line.
• A 600 W inverter load on a 12 V battery draws 50 A from the battery, before inverter losses, which is why low-voltage systems need thick cables.

The 80% rule for continuous loads

In North America, electrical codes generally size circuits so that a continuous load, one that runs for three hours or more, uses no more than 80% of the breaker rating. In practice that means a 15 A circuit is planned for about 12 A of continuous load and a 20 A circuit for about 16 A. The calculator shows the minimum circuit rating for a continuous load at this level, or the continuous watts a given circuit can carry. Other countries use different rules, and appliances are built to match standard outlets, so always follow your local code and the appliance instructions, and ask a licensed electrician before adding or changing circuits.

What voltage do you have?

• US and Canada: 120 V for standard outlets; 240 V for dryers, ranges, water heaters and EV chargers; 208 V three-phase in many commercial buildings.
• UK, Europe, Australia, most of Asia and Africa: 230 V (UK and Australia often quoted as 240 V) single-phase; 400 V three-phase.
• Vehicles, boats, RVs and off-grid systems: 12 V, 24 V or 48 V DC.

Power factor in practice

Most nameplates on motors give either the power factor or both watts and volt-amperes (VA). Power factor = W ÷ VA. Electric motors commonly run with power factors below 1, so the current is higher than watts ÷ volts suggests. If you only know VA, use PF = 1 with the VA figure: VA ÷ V gives the current directly.

Volts, amps and watts in plain words

Voltage is the electrical "pressure", current (amps) is how much charge flows, and watts are the rate of energy use. For the same appliance power, a higher voltage needs less current, which is why high-power appliances in North America use 240 V circuits and why long cable runs in 12 V systems lose so much energy. Cable size depends on current, not watts, so converting to amps is the first step in choosing wire and fuses.

Common mistakes

• Using phase-to-neutral voltage in the three-phase formula. Use line-to-line voltage (for example 400 V, not 230 V) with √3.
• Ignoring inverter losses on DC systems. A 600 W AC load at 90% inverter efficiency draws about 667 W, or about 56 A, from a 12 V battery.
• Using starting current for running loads. Motors draw several times their running current for a moment when they start; see the generator size calculator.

For planning only. Electrical work should follow local codes and be done by a qualified electrician.

Frequently asked questions

Sources

• National Fire Protection Association. NFPA 70, National Electrical Code: branch-circuit ratings and continuous loads.
• International Electrotechnical Commission. IEC 60038: Standard voltages.
• US Department of Energy, Energy Saver. Determining electric motor load and efficiency.

Last reviewed 27 October 2026. Results are estimates; see our disclaimer.