Electrical appliances

Power is the work done or energy transferred over a period of time. We can also represent power using the equation below.

  • Power – Measured in watts (W)
  • Work done – Measured in joules (J)
  • Time – Measured in seconds (s)

This means 1 Watt = 1 Joule per second

We can also calculate the energy transferred by rearranging the equation, to form:

Electrical Appliances

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 of 1850-2000 W.

If we wanted to calculate the energy this electrical appliance uses per minute. We would use the equation:

Energy (J) = Power (W) × time (s)

As there is a range, we would need to calculate the energy for both 1850W and 2000W.

  • 1850 W × 60 s = 111000 J
  • 2000 W × 60 s = 120000 J

So the energy used by this appliance per minute will be between 111000 J and 120000 J.

We can also convert this to KJ and the range will be 111 KJ – 120 KJ.

Watts to kilowatts

However, sometimes the energy of an electrical appliance is shown in kilowatts (kW) instead of W, this occurs when the appliance uses large amounts of energy per second.

  • 1 kW = 1000 W

Below is a table showing the conversion between power in W and power in kW with different appliances.

Electrical appliancePower in WPower in kW

Cost of energy

Energy is normally measured in joules (J). However, companies use the unit:

Kilowatt-hour (kWh)

So they will charge per kWh of energy used.

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:

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.

Saving energy

To save energy, you can use different appliances that have the same performance but a lower power rating. You can also use appliances for shorter periods of time.

Total energy cost

To calculate the cost of the total energy used: