What is the formula for amps to watts?

For DC circuits the formula is P (watts) = V (volts) times I (amps). For AC single-phase circuits you also multiply by the power factor: P = V times I times pf. For AC three-phase circuits the line-to-line voltage and square-root-of-three factor come in: P = 1.7321 times V times I times pf. The calculator applies the correct form automatically depending on which circuit type you select.

What is power factor and why does it matter?

Power factor (pf) is the ratio of real power (watts, the power that does useful work) to apparent power (volt-amps, the product of voltage and current). It ranges from 0 to 1. A purely resistive load such as an electric heater or incandescent bulb has pf = 1.0. Inductive loads such as motors, transformers and fluorescent ballasts have pf below 1, meaning the current drawn is higher than you would calculate from watts alone — the apparent power in VA is always at least as large as the real power in watts.

How do I convert watts to amps?

Rearrange the power formula to I = P divided by V (DC) or I = P divided by (V times pf) for AC single-phase, or I = P divided by (1.7321 times V times pf) for three-phase. For example, a 2,300 W load on a 230 V supply with pf 1.0 draws 2300 divided by 230 = 10 A. Switch the "Solve for" selector to "Amps" and the calculator does this rearrangement for you.

What is the difference between kW and kVA?

kW (kilowatts) measures real power — the energy per second actually converted into work or heat. kVA (kilovolt-amps) measures apparent power — the product of the RMS voltage and RMS current. The two are related by kW = kVA times pf. For a unity-power-factor (resistive) load they are equal. For a motor with pf 0.85, a 8.5 kW motor draws 10 kVA from the supply, so the wiring and switchgear must be rated for 10 kVA even though only 8.5 kW does useful work.

What voltage should I use for three-phase calculations?

Use the line-to-line (phase-to-phase) voltage, which is 400 V on a standard European LV supply or 208 V or 480 V on common North American systems. The three-phase formula P = square-root-3 times V_LL times I times pf already includes the geometry of the three-phase system, so you never need to divide by three or use the line-to-neutral voltage.

How accurate is this calculator?

The calculation is mathematically exact given the inputs. Real-world accuracy depends on how well your measurements match the steady-state RMS values assumed by the formulae. Rapidly varying loads, harmonic-rich currents (switch-mode supplies, VFDs) and unbalanced three-phase systems can all cause the actual power to differ from the formula result, but for most everyday sizing and estimation purposes the answers are reliable.

Amps to Watts Calculator

Converting amps to watts — or working backwards from watts to find the required current — is one of the most common electrical calculations for homeowners sizing a circuit, engineers specifying cable and switchgear, and anyone checking whether an appliance or generator will cope with a given load. This calculator handles all three circuit types: DC, AC single-phase and AC three-phase, with a built-in power factor control for AC loads, and shows the complete step-by-step working so you understand exactly how the answer was reached.

How it works

The relationship between voltage, current and power depends on the circuit type.

For DC circuits the formula is simply:

P (W) = V (V) × I (A)

Power equals voltage multiplied by current. There is no power factor because DC has no phase angle between voltage and current.

For AC single-phase circuits the phase angle between voltage and current waveforms means you must include the power factor (pf):

P (W) = V × I × pf

The product V × I gives the apparent power in volt-amps (VA), and multiplying by pf gives the real power in watts that actually does useful work. A purely resistive load such as an electric heater has pf = 1.0, so watts and VA are identical. An inductive motor might have pf = 0.85, meaning 85% of the apparent power becomes real work and the remaining 15% is reactive power exchanged with the supply without doing net work.

For AC three-phase circuits the three conductors each carry a phase-shifted current, which introduces an additional geometric factor of √3 (approximately 1.7321):

P (W) = √3 × V_LL × I × pf

Here V_LL is the line-to-line voltage (400 V in Europe, 208 V or 480 V in North America). The apparent power is S = √3 × V_LL × I (in VA or kVA). Reactive power follows as Q = √(S² − P²) in VAR.

Worked example

A three-phase air-conditioning unit on a 400 V supply draws 16 A per phase and has a power factor of 0.92:

Apparent power: S = √3 × 400 × 16 = 1.7321 × 400 × 16 = 11,085 VA ≈ 11.1 kVA
Real power: P = 11,085 × 0.92 = 10,198 W ≈ 10.2 kW
Reactive power: Q = √(11,085² − 10,198²) ≈ 4,317 VAR ≈ 4.3 kVAR

The circuit must be rated for 16 A per phase (apparent) even though the unit only consumes 10.2 kW (real). Working the reverse direction: if you specify the load as 10.2 kW and need the current, the tool solves I = P ÷ (√3 × V × pf) = 10,200 ÷ (1.7321 × 400 × 0.92) = 16 A.

Circuit	Voltage	Current	pf	Real power
DC (battery)	12 V	10 A	1.0	120 W
AC single-phase	230 V	10 A	1.0	2,300 W
AC single-phase	230 V	10 A	0.85	1,955 W
AC three-phase	400 V	16 A	0.92	10,198 W

Formula note

All formulae assume steady-state sinusoidal voltages and currents at the fundamental frequency. Non-linear loads (switch-mode power supplies, variable-frequency drives, LED drivers) draw harmonic currents that increase the true RMS current above the fundamental-only prediction. For precise energy metering or harmonic analysis you need a power-quality analyser; for cable sizing and circuit protection the worst-case (apparent) current in amps is what matters, and this calculator provides that figure directly. Everything runs locally in your browser — no data is sent anywhere.