We saw previously that the total energy transferred by an appliance depends on how long the appliances actually switched on for, and the power at which it is operating. We know that power is measured in Watts and generally we can tell that for example a 3 kW electric fire will be far more "powerful" then a 40 W lightbulb. But how do we link together the power of an appliance and the energy transferred to or by it?


The "power" of an appliance is a measure of the energy that it transfers per second:


Where Et is the energy transferred (by electrical work), P is the power of the appliance and t is the time in seconds.


Q. A 2 kW electric kettle takes 1 1/2 minutes to boil 1 L of water. How much energy is transferred by the kettle during this process?


A. This is a straightforward "plug and play" type of question we are given the equation, and the values required and there is no need to transpose or rearrange the equation first.


Note that this question is about as easy as you're going to get, however you do need to be careful to make sure that you convert your resources into the appropriate units, for example 2 kW is 2000 W and 1 1/2 minutes is 90 seconds.