is a unit of energy, but this is a very small amount of energy. For example a 60W light bulb left on for 5 hours (approx 1 evening) will use 1,080,000J. It is worth noting that a 60W light bulb does not use much energy when compared to heating systems like ovens and showers.
Electricity bills can be issued as infrequently as once every 3 months. A larger unit of energy is therefore required, in order to make the numbers involved smaller. One unit of electrical energy is given in Kilowatt hours. You then pay a price per unit of electricity.
To work out the number of units an appliance uses in kWh, you multiply the power rating of the appliance in kilowatts by the time it was on in hours. The same equation but with different units:
E = P × t
where:
E is energy transferred in kilowatt-hours, kWh
P is power in kilowatts, kW
t is time in hours, h
One kilowatt-hour is also known as one unit of electricity and is what an electrical company will bill you in. Calculate the cost of electricity used as follows:
Total Cost = Cost per unit × number of units used
The number of units used in your home is measured by your electricity meter. To calculated the amount of electricity used in a period of time, you simply find the difference in meter readings at either end of the period - ie take the smaller number away from the larger (later) one.
Example:
Calculate the number of units of electricity a 60 Watt light bulb will use in 5 hours.
- P = 60W = 0.06kW
- t = 5hrs
Hence, calculate the cost of running the bulb, if electricity costs 8p per unit
