Electrical Appliances

Power is the rate at which energy is transferred by an appliance over a period of time. We can also represent power using the equation below:

An equation expressing the relationship between power, energy, and time. The formula reads: Power (in Watts) equals Energy (in Joules) divided by Time (in seconds).
  • Power – Measured in watts (W)
  • Energy – Measured in joules (J)
  • Time – Measured in seconds (s)

This means that 1 Watt = 1 Joule per second.

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

An equation expressing the relationship between energy, power, and time. The formula reads: Energy (in Joules) equals Power (in Watts) multiplied by Time (in seconds).

Electrical Appliances

All electrical appliances show their power rating in watts, on a label known as an electrical rating plate.

A label from a product with the model number SG620. It specifies a voltage range of 230-240V at 50Hz and a power range of 1850-2000W.

This is a rating plate on an electrical appliance between 1850 W and 2000 W.

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.

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

So, the energy used by this appliance per minute will be between 111,000 J and 120,000 J. If we convert this to kJ, the range will be between 111 kJ and 120 kJ.

Watts to kilowatts

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.

  • 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). Therefore, 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, we use the equation:

An equation illustrating the relationship between energy, power, and time. It reads: Energy in kilowatt-hours equals Power in kilowatts multiplied by time in hours.

Household bills contain information such as the cost per kilowatt-hour, the number of kilowatt-hours used, and the total cost of the energy used.

Saving energy

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.

Total energy cost

To calculate the total energy cost:

An equation describing how to calculate the total cost of energy. It reads: Total cost of energy in pounds sterling equals Energy Used in kilowatts multiplied by Cost of 1 kilowatt-hour.

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