Slacking off exercise

I had been managing to average over 23 of the cross-trainer's calories per minute. In pushing my workouts to be a little harder and longer I had hoped to be drifting up toward 25 calories/minute but my power level seems to have firmly plateaued. So, I shall now push it in the opposite direction: try working out a little less and see if my power level settles at least above 20 calories/minute. After all, the point is to be getting enough exercise to be acceptably healthy.