In this paper, we review and use different methods to measure and compare efficiency scores in energy producing plants. In particular, we use non-parametric and parametric techniques. We focus our attention in electricity producing power plants on eighteen European countries, as well as in thirty energy systems as a whole. This paper also state some results such as that efficiency has widely improved in the period studied, but these positive results are not homogeneous among energy systems or firms. We present some evidence that the greatest part of energy improvement is the consequence of the technological shift and is not necessarily due to alternative factors, such as market integration, increasing competition, or other firm-level decisions.