Electrical Power

P = V × I

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Result
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Formula

P = V × I = I²R = V²/R

Description

Electrical power measures the rate at which electrical energy is converted into another form such as heat, light, or motion. The three equivalent forms are derived by substituting Ohm's Law into the base formula P = VI. The I²R form is particularly useful for calculating resistive heating losses in wires and components, known as Joule heating or I-squared-R losses. The V²/R form is convenient when voltage is fixed and you want to find how power changes with resistance, such as in power supply loading scenarios.

Variables

  • P — Power in watts (W), the rate of energy transfer
  • V — Voltage in volts (V)
  • I — Current in amperes (A)
  • R — Resistance in ohms (Ω)

Practical Notes

Power ratings on resistors indicate the maximum continuous dissipation before damage. Always derate by 50% or more for reliability. In AC circuits, this formula gives apparent power; real power requires multiplying by the power factor.

Related Formulas

Ohm's Law Voltage Divider Current Divider Thévenin Equivalent Norton Equivalent Temperature Coefficient of Resistance

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