I was discussing with a friend recently the possibilities of utilising energy generated during exercise to power common electrical items.
He got quite heated, if you'll pardon the pun, when I explained I thought it would take far too much energy to boil a kettle, for one person to be able to make a pot of tea after a swift workout.
I calculated that if you have an 80 percent efficient mechanism for converting a dropping one-tonne mass into heat, you would need to raise it almost 5 metres in order to boil half a litre of water.
Now call me an unfit slob if you will, but there is no way I'm lifting a tonne through five metres for a cup of tea, cunning effort-reducing pulley mechanisms or no.
I'd be interested to see if anyone else can get to about the same figure, and how. It's entirely possible, in fact I'd go so far as to say likely, that my maths is way out.