In this work the equations that determine the short term optimal point of operation of a power system are obtained from two different perspectives. The first one, optimizing the system from a global point of view. The second one, takes into account the invididual agent’s behaviour which buys and sells electricity at each of the power systems’ busbars. From the comparison of the equations obtained from each case, the prices of active and reactive energy that optimize the system from the global perspective and from the individual agents’ perspective at the same time, are deduced. This leads to the definition of the system marginal price and the nodal factors. An interpretation of these magnitudes is done and the current practices for nodal factor calculations is analysed, looking at possible inconviniencies and contradictions. Finally, a particular case, considering the uruguayan power system is studied.