Watts, Amps & Volts Calculator

What is the watts, amps and volts relationship?

Voltage is the electrical pressure that pushes charge through a circuit, current is the rate at which charge flows, and power is the rate at which energy is used. In a DC circuit these combine in the simplest possible way: power in watts equals volts multiplied by amps. A 12 V device drawing 5 A uses 60 W.

In AC circuits the voltage and current rise and fall together, but they are not always perfectly in step. The power factor describes how closely they line up, and it reduces the real power you get from a given voltage and current. Three-phase supplies add a further factor because three conductors share the load. This calculator handles all three cases, so you can enter any two of power, current and voltage and read the third.

Formulas

Power factor (PF) is the ratio of real power in watts to apparent power in volt-amperes, and it is a number between 0 and 1. A resistive heater is close to 1, while motors and other inductive loads are usually lower. Three-phase formulas use √3 (about 1.732) because the line-to-line voltage of a balanced three-phase supply is √3 times the voltage across each phase, and the three phases together carry three times the power of one.

Worked examples

Single-phase power: a load on 230 V draws 10 A with PF 1. Power = 230 × 10 × 1 = 2,300 W, which is 2.3 kW.

Three-phase power: a motor on 400 V line-to-line draws 10 A at PF 0.8. Power = 1.732 × 400 × 10 × 0.8 = 5,542.56 W, about 5,543 W or 5.54 kW. The apparent power is 1.732 × 400 × 10 = 6,928.2 VA.

Finding current: a 2,000 W single-phase load on 230 V with PF 1. Current = 2,000 ÷ (230 × 1) = 8.6957 A, or about 8.7 A.

These supply voltages are examples only. Common supply voltages vary by country, so enter the value that applies to your installation.

kW vs kVA

Kilowatts (kW) measure real power, the part that does useful work such as turning a shaft or producing heat. Kilovolt-amperes (kVA) measure apparent power, the product of voltage and current regardless of how well they line up. The two are related by the power factor: kW = kVA × PF, and kVA = kW ÷ PF. The remainder is reactive power in kVAR, found as √(kVA² − kW²).

For example, a load of 80 kW at a power factor of 0.8 needs 80 ÷ 0.8 = 100 kVA of apparent power, with 60 kVAR of reactive power. Equipment such as generators and transformers is commonly rated in kVA because it has to carry the full current whatever the power factor is.

Common mistakes

Mixing line-to-line and line-to-neutral voltage. The three-phase formulas here expect the line-to-line voltage. Using the line-to-neutral value gives a result that is wrong by a factor of √3.

Ignoring power factor. Assuming PF 1 for a motor or other inductive load underestimates the current needed for a given power.

Confusing kW and kVA. They are equal only at a power factor of 1.

Treating the result as wiring guidance. This tool gives estimates only. Cable, fuse and breaker sizing depend on standards, temperature, cable length and other factors, so consult a qualified electrician.