An integrated approach to the assessment of energy systems behaviour with regard to energetic, economic and environmental viewpoints is suggested. For each aspect, suitable performance indicators are defined, evaluated and then normalised in order that the same numerical value is calculated for qualitatively equivalent performances even if related to different aspects; application of these indicators can address policy makers, designers and plant operators towards a fair compromise among such important aspects. In this paper the method is applied to different types of plants (thermal power, gas-steam combined, co-generative, geo-thermal power, hydroelectric) and results are compared and discussed. The procedure can be effective in both determining standard performances for each class of plants and analysing the deviations of a particular plant from the average behaviour of its class.A primary version of this paper was presented at the ECOS’00 Conference in Enschede, July 5-7, 2000