In the paper alternatives for expansion of the Macedonian electricity supply system are presented. MESSAGE tool is used for modelling the system, for developing the scenarios, to analyse cost optimal energy pathways and to determine the optimal electricity generation technology mix. The calculations for the period 2020-2050 are made in a 5-year interval. Two scenarios are developed: Scenario 1 – BAU (Business as Usual) scenario, where the current energy structure prevailed, with trends like the current development. The electricity demand is satisfied by coal-fired thermal power plants, gas-fired thermal power plants, hydro power plants and renewable power plants. Scenario 2 – Green scenario, where gas-fired thermal power plants and nuclear power representative with small modular reactors are base load technologies, and also intensive construction of production capacities on renewable energy sources is forced. By making further analysis, the possibility of flexibility in the construction of new facilities within the interval, economic indicators, and appropriate impact on the environment are obtained. In order to make an economic analysis for a particular technology, calculations are made for parameters that are standard for projects, such as: net present value, benefit – cost ratio, payback period and internal rate of return. The environmental impact is analysed of aspect of CO2 emissions.