In addition to equipment and facilities among consumers, the efficiency of power generation is a very important aspect of European energy policy. Increasing efficiency is a key issue, in which case the electrical efficiency is increasingly used as a criterion of the efficiency of power generation. One of the reasons for this is the easier legitimization of new power plants, as these require significantly less raw materials with higher efficiency and cause less environmental damage while producing the same amount of power. In order to guarantee the energy supply and to stem the rise in import dependence, considerations about the increase in the efficiency of power plants are required. The central aim of this work is to demonstrate the efficiency criteria of power conversion systems in order to reduce the primary energy consumption and CO2 emissions significantly. In addition, the development of power plant efficiency of some power plant technologies of the recent years is presented. It shows, for example, that coal fluidized bed combustion in the power range of 15-200MW with the use of CHP reaches an electrical net efficiency of 24-28% and an thermal- from 62-64%, which leads to a fuel efficiency of 75-85%. Gas turbines with HRSG in CHP mode have a net electrical efficiency of 30-36%, a thermal- from 44-50% and thus ultimately a fuel efficiency of 70-84% in the power range of 5-25MW power plant capacity. In order to provide a common starting point, at first it is necessary to develop precise definitions of the used terms as gross- and net efficiency, fuel efficiency or minimum efficiency as well as guidelines for their determination. The most common power plant technologies in Austria are examined and, if required, a classification by energy source and power class. For photovoltaic and wind energy, the criteria performance ratio and reference yields are elected, thus moving away from the conventional way of measuring the efficiency by means of electrical efficiencies. In both cases this leads to a neutral evaluation of the system, as the evaluation using efficiencies in these cases would not be productive. Furthermore, it is documented, in which countries regulations/guidelines for efficiency of various power plant technologies already exist. It turns out, that only a few regulations/guidelines exist, which take the efficiency as a criterion of efficiency of power conversion systems into account. Often the minimum efficiency of a power plant is set to the State of the art, but this state is often not quantified. Ultimately, the options of the repowering of selected power plants to the state of the art are analyzed in terms of a ecological (CO2 emissions) and economic perspective. An analysis of the associated reduction in CO2 emissions and a sensitivity analysis complete the work.