The concept of the coupled energy and environmental transition is presented and discussed. Two transition goals are considered: carbon-free electricity generation and sustainable development. Appropriate indicators are used to indicate the transition's development. The intermittent renewable energy sources are considered for increasing carbon-free electricity generation. Presented concept shows the relation between improvements in forestry and possible further use of coal for electricity generation with simultaneously fulfilled imposed limit of carbon dioxide emission. The relation is given in the analytical form. The aim is to contribute to the options for sustainable development of undeveloped and developing countries. The example of Serbia is also presented and discussed.