Electrical Power

The following equation was introduced in Module 1.

Power = W / T

Where W = Energy transferred (J)
t = Time (s)
Power (W)

This can be rearranged to give:

Energy transferred = Power x Time

Module 2 introduces new equations which allow the power of the circuit to be calculated from values for the voltage, current or resistance.

Power = VI

Power = I2R

Power =

Thus combining Energy transferred = Power x Time with Power = VI
it is given that:

Energy = VIt


Domestic use of energy
Another unit for energy is the kWh, it must be noted that this is not an SI unit. This is mainly used domestically in particular on energy bills. The equation is as follows:

Energy transferred (kWh) = Power (kW) x Time (h)

An electrical device has a power rating of 12 W. Calculate the current when a potential difference of 230V is supplied. How much energy is transferred in a day in kWh?

Power = VI
12 = 230I
I = 0.0522 A

Energy = VIt
Energy = 12 x 12
= 144 kWh