Electrical appliances

Power is the rate at which work is done or energy is transferred over time. specifically refers to how quickly that work is done or how quickly energy is transferred.

The equation for power is:

  • 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. For example, a rating plate on an electrical appliance might display a range of 1850-2000 W.

If we want 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

Therefore, the energy consumed by this appliance per minute ranges between 111,000 J and 120,000 J. This range can also be expressed in KJ, which would be 111 KJ to 120 KJ.

Watts to kilowatts

However, sometimes the power of an electrical appliance is represented in kilowatts (kW) instead of W. This typically occurs when the appliance has a high energy consumption 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). As a result, they will charge per kWh of energy used. A kilowatt-hour represents the amount of electrical energy consumed by an appliance using 1,000 watts for one hour.

To work out the amount of energy in kWh used by an electrical appliance, 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.

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, use this equation: