This page shows how power (P), voltage (V), current (I) and resistance (R) are related.

  • Power P is measured in Watts - this is the energy per second dissipated (given out) by the component (heat, sound, light, movement, etc)
  • Voltage V is measured in Volts - this is the voltage across the component
  • Current I is measured in Amps - this is the current through the component
  • Resistance R is measured in Ohms - this is the resistance of the component

 

Power Calculation

In the diagram, the power in Watts is dissipated as heat by the resistor.
To prevent overheating, the resistor power rating must be sufficient.

 

P = I V

P = V2 / R

P = I2 R

 

Alternating Currents

To calculate RMS power, use RMS values for voltage and/or current.

Prms = Irms Vrms

Prms = Vrms2 / R

Prms = Irms2 R

 

Deriving Power Formulas

Ohm's law states that current, voltage and resistance are related in the following three ways ...

  • R = V / I
  • V = I R
  • I = V / R

P = V I

V = IR

hence

P = I2 R

P = V I

I = V / R

hence

P = V2 / R