For example.... wind turbines are primarily listed by their wattage (1.5MW etc.) Presumably their output is varied according to rotational speed, so I'm guessing that the rating is simply a guide. However, for a given mass, at a given height; due to standard gravity, can we state that this mass, on falling can deliver a maximum amount of watts of power. Eg. Work = force times distance. Power = work divided by time. Work = 100kg @ 100m = 98,000 joules Time = 100m freefall = 4.5s Power = 21.8KW But that is freefall, and nothing is being driven. If a chain and gearbox, was attached to the mass, that then rotated a generator....... the rate of descent would be very slow. Leaving aside friction.... work must be done to turn the generator? How then do we predict the output of the generator? Eg. the mass takes 4 times as long to travel 100m Is the power output 21.8KW/4 = 5.45KW for a period of 18s ? Or, to put this into a 1KW element fire: we could only get 98s of use, from 100kg @ 100m. Surprisingly little, if the above is true. But is it true? Is 21.8KW the maximum we can get out of 100kg @ 100m?