Using the gravitational potential energy formula

E = mgh we can calculate that 1 step will take

0.3 * 650 = 195 Joules, where 650 = mg as weight is just mass multiplied by the acceleration due to gravity.

So 40 steps is just 40 * 195 = 7800J

Power is the rate of energy, so we have 7800/60 = 130 Watts as there are 60 seconds in a minute.