Energy systems combine the sources of energy, the means for converting these sources to usable forms, the distribution devices and procedures, the using community and the ways it employs energy, and the surrounding natural and economic environment. Thus, energy systems are socioeconomic systems whose analysis includes various sciences with different limitations to scientific findings. While their physical behavior may be modeled on the computer with sufficient precision at least on a local or regional level the parameters necessary for economic modeling are subject to large principle uncertainties. Qualitative parameters may enter the analysis via model restrictions or exogenous assumptions, which inevitably involve personal views. Solving the global energy problem is an urgent task that cannot be postponed until a provable scientific solution is found, especially since such a solution may never emerge. In this situation systems analysis based on energy models may support the necessary decisions by enhancing the understanding of energy systems However, such models will not lead to correct or validated quantitative results, but to reasonable qualitative suggestions.