The power dissipated in a single component in a circuit is calculated in one of three ways ...

Power = Volts x Amps

Power = Volts2 / Resistance

Power = Current2 x Resistance

In a circuit with alternating currents, use the RMS current and voltage instead.

The voltage is measured across the single component.

The current is measured through the single component.

In a circuit with many components, each component contributes to the power consumption but there is an easier way to measure the whole circuit power consumption. Measure the whole circuit power supply voltage. Measure the current taken by the whole circuit and carry out the calculation in the usual way (as shown avove).

Example

	The load current is 50mA. The load voltage is 4V. The power supply voltage is 9V. The power supply current is 100mA. Calculate the power dissipated (given out as heat) by a) The Load b) The Resistor c) The Zener Diode d) The whole circuit.
a) Pload = Vload x Iload = 4 x 50e-3 = 200mW   b) Pr = Vr x Ir = 5 x 100e-3 = 500mW   c) Pz = Vz x Iz = 4 x 50e-3 = 200mW   d) Pwhole = Vwhole x Iwhole = 9 x 100e-3 = 900mW   Notice that the 900mW entire circuit power is the same as the separate component powers added together.