Comparing power ratings and energy values

This lesson covers: 

  1. How to calculate energy transferred based on an appliance's power rating
  2. Comparing energy usage of appliances with different power ratings
  3. Finding the energy content of food from nutrition labels

Calculating energy usage from power ratings

The power rating of an appliance tells you how much energy it uses per second.

This allows you to calculate the total energy an appliance transfers.

Use the equation:

Energy (kWh) = power (W) × time (h)


The higher the power rating, the more energy used over a time period.

Calculating energy usage example

Comparison of energy usage between 1.5 kW and 4 kW heaters used for 1.5 hours.

For example we can compare the energy used by a 1.5 kW and a 4 kW heater when they are used for 1.5 hours.

The 4 kW heater uses 3.75 kWh more energy.

Higher power ratings cost more to run as more energy is transferred.

Finding food energy values

  1. Food contains energy measured in kilojoules (kJ).
  2. Nutrition labels state food energy contents per 100 g.

Comparing food labels allows assessment of energy intake. As energy intake impacts health, it's important to be aware of energy consumed.