A method of energy saving is applied to a mobile communications network comprising at least two radio access technology systems (102; 104) having a plurality of base stations (108-118). The method comprises the steps of: assigning (136a; 136b) respective priorities to the radio access technology systems; and an energy saving server (136a; 136b) instructing, on the basis of the priorities, a selected radio access technology system to reduce the operational state cells in a group of cells. This reduction occurs if the selected radio access technology system and/or another radio access technology system is able to maintain service to served mobile terminals.