What is the formula for electrical power?

Electrical power can be calculated three ways depending on which quantities you know: P = V times I (voltage times current), P = I squared times R (current squared times resistance), or P = V squared divided by R (voltage squared divided by resistance). All three formulas give the same result for a purely resistive circuit.

How do I convert watts to kilowatts?

Divide watts by 1,000 to get kilowatts. For example, 1,200 W equals 1.20 kW. Kilowatts are the standard unit used on electricity bills, and kilowatt-hours (kWh) measure the energy consumed over time.

How much does it cost to run a 1200 W appliance?

A 1,200 W appliance running for one hour uses 1.2 kWh. At a typical New Zealand residential electricity rate of around 35 cents per kWh, that is about 42 cents per hour or roughly $1.00 per day if run for 2.5 hours. Use the calculator to adjust for your actual rate and usage time.

Electrical Power Calculator

Electrical power is the rate at which energy is transferred or consumed in a circuit, measured in watts (W). There are three equivalent ways to calculate it depending on which circuit quantities you already know. If you have both voltage and current, use P = V times I. If you have current and resistance but not voltage, use P = I squared times R. If you have voltage and resistance but not current, use P = V squared divided by R. All three formulas are equivalent for resistive circuits and will give the same result when the values are consistent. You choose the formula you want, enter the two required inputs, and the calculator returns power in watts, converts it to kilowatts, and estimates your daily energy cost based on hours of use and your electricity rate. This is useful for a wide range of practical tasks: working out how much power an appliance, motor, or heating element draws; sizing fuses and circuit breakers; comparing the efficiency of different devices; calculating electricity running costs; and designing power supplies. For the default example of 240 V and 5 A, the power is 1,200 W or 1.20 kW, the typical load of a small kitchen appliance. Results assume a purely resistive load. AC circuits with inductive or capacitive loads require power factor correction, which is handled by the Watts to Amps calculator on this site.

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Formula

Voltage (V)

Current (I)

Resistance (R)

Ω

Hours of use per day

Electricity rate

/kWh

1200.00 W

electrical power

Kilowatts1.20 kW

Daily energy2.40 kWh

Daily cost$0.84

How it works

Select the formula matching the values you have. P = V times I multiplies voltage by current directly. P = I squared times R squares the current and multiplies by resistance; V is not needed. P = V squared divided by R squares the voltage and divides by resistance; I is not needed. The calculator disables the irrelevant input for the chosen formula. Kilowatts are watts divided by 1,000. Daily energy in kilowatt-hours is kilowatts times hours of use. Daily cost is daily energy times your electricity rate per kilowatt-hour.

Worked example

A household circuit carries 5 A at 240 V. Using P = V times I: power = 240 times 5 = 1,200.00 W = 1.20 kW. Running for 2 hours a day uses 2.40 kWh. At $0.35 per kWh, the daily cost is $0.84. These match the default values pre-filled above.

Related calculators

Ohm's Law Calculator: solve for voltage, current or resistance using V=IR.
Watts to Amps Calculator: find current from power and voltage for DC or AC loads.
Amps to Watts Calculator: convert current and voltage to power for DC or AC circuits.
Percent Error Calculator: compare measured power readings against expected values.

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