Since the first oil shock, the energy sector has experienced a wide range of influences that have greatly influenced energy analysis and modelling activities. Energy models were however not developed for the same purpose; some were concerned with better energy supply system design given a level of demand forecast, better understanding of the present and future demand–supply interactions, energy and environment interactions, energy-economy interactions and energy system planning. Well-known modelling tools are LEAP(Long-range Energy Alternative Planning Model)- a popular model, MEDEE (Model for Evaluating Demand for Energy) family of programmes and MAED (Model for Analysis of Energy Demand). By using the above mentioned modelling tools, countries set their energy policies, scenarios and their strategies. Coal was displaced by natural gas in the UK to a large extent and in Spain and Netherlands to a lesser extent. In Italy, fuel–oil based generation which was the dominant form of power in the mid-1990s was replaced by natural gas. In the British case, the electricity generation mix for 2010 was as follows: 38% came from natural gas, 36% from coal, 22.5% from nuclear and the rest from renewable sources including hydropower. Natural gas consolidated its position as the leader in the Netherlands during this period. In 2011, dependence on fossil fuels in electricity generation remained very high in the Netherlands (88%), Italy (79%) and the UK (above 70%). In 2011, U.S. energy supply is 83% fossil fuels; demand is broadly distributed among the major sectors. And in 2010, U.S. electricity generation was 70% fossil fuels, 20% nuclear, and 10% renewable. By the end of 2011, Turkey’s gross electricity production has reached 228.431,02 GWh, i.e. 102.130,71 GWh (44,7%) natural gas, 668 GWh (0,3%) of geothermal, 52.078,04 GWh (22,8%) hydro-electric and 4.726,02 GWh (2,1%) wind sources. End of 2011, Turkey’s installed power reached 49.126 MW. Turkey’s electricity generation is expected by 2020 to reach 499 TWh with an annual increase of around 8% according to the higher demand scenario, or 406 TWh with an annual increase of 6,1% according to the lower demand scenario. By 2020 EU-27’s electricity generation details is as follows: Wind will reach 477 TWh, Hydro will reach 384 TWh, Photovoltaic will reach 180 TWh, Biomass will reach 250 TWh, Geothermal will reach 31 TWh, Solar thermal electricty will reach 43 TWh, Ocean will reach 5 TWh, i.e. by 2020 a share of over 40% of renewables in electricity production is expected. By making correct scenarios with using the correct modelling tools, Turkey will reach 2023 energy target easily by correct annual energy investment rate, i.e. “By 2023, Turkey’s main target is to increase the total installed energy power to 100.000 MW and the share of renewable resources is to increase at least 30 percent of total production, i.e. 20,000 MW installed capacity for wind, 1.000 MW installed capacity for geothermal and an additional installed capacity of 5,000 MW for small hydro and to have at least 3 operating nuclear power plants.”