In Greece, there is not any unique system for the classification of the mineral commodities reserves/ resources. The mining companies and the Institutes have their own classification systems, which are mainly based on the other countries classifications, such as the American, the Russian and French systems. However, the correlation of these classifications is difficulty implemented. The mineral commodities reserves/ resources of Greece are categorised as "proven", "possible", and "probable" reserves. The Limits between the three categories are not clearly indicated, therefore the results from their evaluation may not be consider reliabIe. However, the "economically recoverable reserves" is another category which is broadly used from the mining companies to characterise their deposits even though the reserves are estimated by each individual company using its own criteria and system. In the past the Institute of Geology and Mineral Exploration and other Organisations proposed several classification systems for the reserves/resources of the country, but no one has been adopted by the State. On the contrary, as it concerns the solid fuels, the Institute of Geology and Mineral Exploration has already established and apply a Classification System for the estimation of the lignite reserves of the country. This system has been accepted by the State and the Public Power Corporation.
The solid fuels of Greece are mainly consist of lignite, while there are some small-sized uneconomic occurrences of hard coal in three areas of the country and the peat deposits that have not been yet exploited. The Lignite production is around 58 million tons per year and contributes to the country's energy consumption with approximately 30% and to the electricity generation with 75%. About 95% of the lignite production is extracted from the Public Power Corporation open pit mines which belong to the State.