This work provides na approach that makes possible an economical comparison of alternative System designs yielginh the desired reliability properties. The selected probabilistic method was the frequency and duration method. An extremely important aspect of this approach is the introduction of a varying environmental condition associated with the System Components. The chosen method although difficult to apply to a complicated system, composed of several generating plants and interconnections, is quite simple to employ in systems which can be reduced to one equivalent element between the source and the load, by Series-Parallel reductions. As in most of the casas, the basic system component paramenters are not available, with an adequate degree of confidence, the developed program permits the claculation of various measures of reliability in power systems for a wide range of influent values in this calculation, for the investigated alternatives. For each of those measuresm results a cost associated to each alternative, considering the initial investiments as well as the costs owing to energgy not sold during interruptions.