Volts, amps, ohms, watts — any two give the rest

Voltage equals current times resistance — V = I × R. Rearranged, current is voltage over resistance and resistance is voltage over current. Combined with P = V × I it gives the four quantities this calculator solves between, so any two of voltage, current, resistance and power determine the other two.

P = V × I — power in watts is voltage times current. Substituting Ohm's law gives the two other useful forms: P = I² × R when you know the current and resistance, and P = V² ÷ R when you know the voltage and resistance. All three are the same equation.

For single-phase, kW = volts × amps × power factor ÷ 1000. For three-phase, multiply by the square root of three as well: kW = 1.732 × volts × amps × power factor ÷ 1000. A 20 A single-phase load at 240 V and 0.85 power factor is about 4.1 kW. Leaving the power factor out overstates the real power by 15–20% on most motors.

Power factor is how much of the current is doing real work rather than circulating as reactive power. Use 1.0 for resistive loads such as heaters, water heaters and incandescent lighting. Motors, compressors and older fluorescent gear typically run 0.8 to 0.9. The nameplate is the right source — the value on the equipment beats any rule of thumb.

kW is real power, the part that does work. kVA is apparent power, the total the supply actually has to deliver, and it ignores the power factor. kVA is the larger of the two on any load with a power factor below 1, and it is the figure that sizes generators, transformers and supply cable — sizing a generator on kW alone will leave it short.

It works directly for purely resistive AC loads such as heaters and incandescent lamps. Once there is inductance or capacitance — any motor, transformer or electronic supply — voltage and current fall out of step, and you need impedance rather than plain resistance, plus a power factor in the power equation. That is what the AC mode in this calculator handles.

Get the power in kilowatts, multiply by the hours it runs to get kilowatt-hours, then multiply by your rate per kWh. A 2.4 kW load running 5 hours a day is 12 kWh, so at 18 cents that is $2.16 a day. The calculator shows kWh for 24 hours of continuous running as a starting point.