This investigation provides an approach towards identification, selection or construction of certain unconstrained controls called Carnot variables which are particularly suitable for easy determining of power limits in various power systems. Applying these controls we develop a unified analysis of how various transfer phenomena (heat, mass and electric charge transfer) effect power limits in various energy converters, such as thermal, chemical and radiation engines and fuel cells. We present diverse pseudo-Carnot structures for converters' efficiencies and apply them in estimating irreversible power limits in steady state systems. Power limits are determined for steady thermal systems propelled by differences of temperatures and for steady chemical systems driven by differences of chemical potentials. Radiation engines are treated as systems described by Stefan-Boltzmann equations. We show that both chemical and electrochemical energy generators (fuel cells) satisfy similar principles of modeling and apply similar schemes of power evaluation as thermal machines. Based on a systematic application of Carnot controls (as variables which satisfy identically the internal entropy constraint) we construct, in fact, a methodologically novel, coherent approach to the family of power systems. Such approach has the virtue of reducing the number of controls and is superior to the traditional approach, which works with an enlarged number of traditional constrained controls. We believe that, because of its formal lucidity, the new approach also improves our understanding of the role of various energy transfer phenomena in power systems. (C) 2010 Elsevier Ltd. All rights reserved.