Power is the energy transferred by an appliance over a period of time. We can also represent power using the equation below:
This means that 1 Watt = 1 Joule per second
We can also calculate the energy transferred by rearranging the equation, to form:
All electrical appliances show their power rating in watts, on a label known as an electrical rating plate.
This is a rating plate on an electrical appliance between 1850 W and 2000 W.
If we want to calculate the energy this electrical appliance uses per minute, we use the equation:
Energy (J) = Power (W) × time (s)
As there is a range, we need to calculate the energy for both 1850 W and 2000 W.
So the energy used by this appliance per minute will be between 111000 J and 120000 J. If we convert this to kJ, the range will be between 111 kJ and 120 kJ.
However, sometimes the energy of an electrical appliance is shown in kW (kilowatts) instead of W, this occurs when the appliance uses large amounts of energy per second.
Below is a table showing the conversion between power in W and power in kW with different appliances.
|Electrical appliance||Power in W||Power in kW|
Energy is normally measured in Joules (J).
However, companies use the unit:
This is a measurement of electrical energy equal to the use of 1000 watts, by an electrical appliance, per hour.
To work out the amount of energy in kWh used by an electrical appliance, we use the equation:
Household bills contain information such as the cost of each kW per hour, the number of kW hours used, and the total cost of the energy used.
To save energy, use different appliances that have the same performance but a lower power rating. You can also use appliances for shorter periods of time.
To calculate the cost of the total energy used: