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Electric Power (P = VI)Free online science calculator — instant results, no sign-up

Calculate electrical power from voltage and current.

Inputs

V
A
Power
1,200 W
In kilowatts1.2 kW
Energy per hour1.2 kWh

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About the Electric Power (P = VI)

The Electric Power (P = VI) calculator multiplies voltage by current to estimate electrical power in watts and kilowatts. It also shows the energy consumed over one hour if that power remains constant. With the default voltage of 120 V and current of 10 A, the results are 1,200 W, 1.2 kW, and 1.2 kWh over one hour.

Voltage (V)
The electrical potential difference across the device, measured in volts (V), with a default value of 120.
Current (A)
The electric current flowing through the device, measured in amperes (A), with a default value of 10.

Formula and method

P = V × I; P_kW = P / 1000; E = P_kW × t, where t = 1 hour. With the default inputs: P = 120 × 10 = 1,200 W; P_kW = 1,200 / 1000 = 1.2 kW; E = 1.2 × 1 = 1.2 kWh.
  • PElectrical power in watts (W).
  • VVoltage in volts (V).
  • ICurrent in amperes (A).
  • P_kWElectrical power expressed in kilowatts (kW).
  • EEnergy consumed during the assumed operating time, in kilowatt-hours (kWh).
  • tOperating time in hours, fixed at one hour for the energy output.

Multiply the entered voltage by the entered current to obtain power in watts. Divide that result by 1,000 to express the same power in kilowatts. Multiply the kilowatt value by one hour to estimate energy consumption for one hour of continuous operation.

Worked example

These are the calculator's starting values and the results it actually produces for them.

Inputs

  • Voltage120 V
  • Current10 A

Results

  • Power1,200 W
  • In kilowatts1.2 kW
  • Energy per hour1.2 kWh

How to use the Electric Power (P = VI)

  1. 1Enter the voltage across the device in Voltage (V); keep 120 for the default example.
  2. 2Enter the current through the device in Current (A); keep 10 for the default example.
  3. 3Read the Power result: 120 V × 10 A = 1,200 W.
  4. 4Read In kilowatts: 1,200 W ÷ 1,000 = 1.2 kW.
  5. 5Read Energy per hour: operating continuously at 1.2 kW for one hour consumes 1.2 kWh.

Reading your result

Power reports the rate of electrical energy transfer in watts: 1,200 W for the default inputs. In kilowatts expresses that same rate as 1.2 kW, not a separate quantity of energy. Energy per hour reports the energy used during a full hour at that constant power: 1.2 kWh. Although the kilowatt and one-hour kilowatt-hour outputs have the same numerical value, their units represent different quantities.

Limitations

P = VI gives power directly for steady DC operation and real power for a single-phase AC load only when RMS voltage and current are used and power factor is 1. For other single-phase AC loads, real power is P = V_rms × I_rms × power factor; multiplying RMS voltage and current alone gives apparent power in volt-amperes (VA). This tool does not account for power factor, three-phase wiring, changing loads, startup surges, or efficiency. The one-hour energy estimate assumes continuous operation at the calculated power, so cycling or varying loads can use a different amount of energy. The result alone cannot determine safe wire, fuse, or breaker sizing.

Common questions

Why do 120 V and 10 A produce 1,200 W?+

The calculator uses P = V × I, so 120 × 10 = 1,200 W. Dividing by 1,000 converts this to 1.2 kW.

Does Energy per hour mean the device always consumes 1.2 kWh?+

No. With the default inputs, 1.2 kWh is the energy consumed only if the device draws 1.2 kW continuously for one hour. Shorter operation or periods when the device switches off reduce consumption.

Can I use this calculator for an AC appliance?+

Yes, as a real-power calculation when the entered values are RMS voltage and current and the appliance has a power factor of 1. For a motor or electronic appliance with a lower power factor, voltage times current gives apparent power, and you need the power factor to calculate real watts.

Should I enter the circuit breaker's current rating?+

Enter the device's actual operating current, not the breaker rating. A breaker rating is a protection limit, not a measurement of how much current the device draws; a device nameplate current may also differ from its actual draw during use.

Does the calculator save my information?+

Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.

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Results are estimates for informational purposes and are not financial, medical or legal advice.