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Electric energy

There is a simple formula for electric power : P = U I

Power is usually defined as energy divided by time

P = E / t , so that energy is power times the time,

E = P t

E=energy (joules) , P = power (watts), t = time (seconds)

It means that the faster energy can be transferred, the higher the power.

This formula is exactly the same in the context of mechanics, or any other branch of physics.Example: a racing car can goes from London to Brighton in 30 min, while an ordinary car takes 45 min. The energy needed for the journey is the same in both cases, but the racing car developed more power because it used less time.


- An electric shower has 1500 W. If it is used during 200 seconds, how much energy has been used* ?

E = P t = 1500*200= 300 000 joules = 300 kJ (kilojoules)

The amount of energy billed by electricity companies usually is measured in kWh (kilowatthour). That means, if you use an appliance that has a 1kW power during 1 h, you consumes 1 kWh.

-How much energy is used if you use a 2 kW electric shower during half an hour? Please give the answer in kWh.

E = 2000*0.5 = 1000 = 1 kWh

-Considering that the a kWh is charged at £ 0.10, how much does it cost to run a 1000 W kettle for 30 min?

Firstly, you calculate the energy used, in kWh:

E = P t = 1 kW * (1/2) h = 0.5 kWh

Now you find the cost:

0.5 * £ 0.10 = £ 0.05, ie, 5 pence.


--> Examples on electric power

*It is important to remember that energy cannot be created or destroyed (or used up); it is also converted from one form to another. The expression used or consumed has been used in this text because it is common in the context of electricity usage and billing.