Does anyone know how mean power is calculated for the vertical jump calculator in exrx.net.
I can't speak for the actual calculation used in the calculator, but
power=work/time or forcexdistance/time
In this case, force is equal to your weight, distance is the height of the jump and time is calculated from the height and gravity.
As mentioned here, to calculate power in Watts in a physical way:
Power is energy dissipated per second.
Energy is work done.
Work done is force x distance.
Force is your body-weight in kilograms multiplied by upward acceleration.
Upward acceleration would need to be in excess of 9.81m/s/s.
You could work out the minimum power produced by:
Multiplying body-weight by 9.81 then multiply by change in height. With this answer divided by the time taken.
To get the absolute power you would need to know your final upward velocity.
A simple although not entirely accurate way would be to calculate the gain in potential energy, which is the (change in height) x (acceleration due to gravity) x (body-weight). Then divide the potential energy by the time taken to produce the change in height.
Example: Person of weight 100 kg. Gravity is 9.81m/s/s. Change in height 1m. Time taken 0.5s.
100 x 9.81 x 1 = 981 Joules
981 / 0.5 = 1962 Watts