Solving power equations
Electrical power is a measurement of output, or the rate of work. Unlike EMF, current flow and resistance, electrical power is not a direct measurement. Nor is it a measure of power produced; rather it is a measure of the amount of power consumed. If an electrical component has a rating in Watts, it will be a measurement of power consumption. A light bulb has a power rating in Watts, but the rating is not an indication of brightness; it is a measure of power consumption.
The symbol for power is P and its unit of measurement is Watts (W). To calculate power, the current flow in the circuit is multiplied by the EMF. So, P = I x E or Watts = Amps x Volts, as is demonstrated by the diagram below.
If a 12 V circuit with a single light has a current flow of 5 A, then applying the formula will yield:
The power consumed by the circuit is 60 W and the bulb would carry a rating of 60 W.
This rule can be applied to any circuit where the EMF and current flow are known. However, if the values of EMF or current flow are not known, then Ohm's Law can be used to determine their value.
As an example: E = I x R, so by expanding P = I x E it can be said that P = I x (I x R) or P = I² x R.
Similarly, by applying I = E/R, then P = (E/R) x E or P = E²/R.
