I hope I can explain my question clearly enough for someone to answer. Suppose we wish to estimate the power required to accelerate a space craft from say 0.9c to 0.9999c in a certain time. By basic definition: Power = Change in Energy / Change in Time. Due to special relativity, there is a difference in these quantities as measured by the stationary earth and the space craft. The classical change in energy would be m*v^2 /2, while the relativistic energy would include the term for increase in relativistic energy. The time dilation effect may also be directly calculated. So to figure the power, which energy (classical or relativistic) and which time (earth or spaceship) should be used as seen on (a) earth, or (b) the spaceship. Are these powers equal?