Dr Matt Little sometimes works with us on projects at phenoptix and does some really very interesting work with the rest of his time. He has a great blog where I just spotted this post:
"Its very difficult to explain the concepts of power and energy. People often ask me "Can I power my kettle with that?".
So to try and put the energy in a cup of tea into perspective I built the 'Cuppa-meter'.
The 'Cuppa-meter' shows a cup of tea which lights up as the energy required for a cup of tea is generated. This can be plugged into a pedal generator and people can 'feel' the energy required to make a cup of tea.
They can either pedal slowly (and hence low power) for a long time, or they can pedal hard (high power) for a shorter time. The end result (the energy) is the same - a nice hot cup of tea (or rather a lit up image of one).
This post gives a few images of the cup of tea and also the calculations used."