In this paper the problem of energy consumption in mechanical systems is approached from an electrical engineering point of view. To achieve this objective classical concepts in electrical networks theory like apparent power, reactive power and power factor have been extended to mechanical systems. This paper focus on the role of springs in mechanical systems to avoid power oscillations between joint actuators and loads. Such oscillations are a major problem because they unnecessarily increases the mean-square value of joint torques and by consequence Joule effect losses in actuators. The minimization of these oscillations is known as "reactive power compensation". The main points of this paper are (i) the deduction of a necessary and sufficient condition for reactive power minimization which can be applied to nonlinear systems with non-sinusoidal periodical motions and (ii) the proposition of "power factor" as an optimization criterion for trajectory generation in robotic systems.