This is a classic.

Energy = Power x Time

So given a question like “How much energy is converted if a 100W light bulb is on for 1 hour?” it’s obvious that you multiply 100W by 1 hour and get the answer 100 Joules.


The formula only works if ALL the bits of it are in the SAME UNITS, in this case SI units. And the SI unit of time is the second, not the hour.

The correct answer is 100W x 1x60x60 seconds = 360,000J or 360kJ

Watch out for this in your exams. Examiners have a nasty habit of throwing non-SI units like hours, centimetres, days etc into the questions. You MUST, MUST convert all the units to SI for the answer to come out correctly.

It’s a really easy way to lose marks, and so easy to avoid. Don’t get caught