Almost 86% of electricity generation in Poland is based on coal and lignite combustion. This share has fallen by 4% in recent years, resulting from a change in the power generation fuel-mix due to an increasing share of renewables, as well as a drop in the export of electricity. The competitiveness of coal in relation to other fuels can be analysed through a comparison of their pricing. Brown coal prices are about 40% lower than steam coal prices, while natural gas is three times more expensive than coal. The greatest increases in prices in recent years have been seen for heavy fuel oil (125%) and natural gas (110%), while coal prices increased by only about 70%. Steam coal competes with brown coal through the price of energy. Currently in the domestic market, this is direct competition due to the obligation imposed on power companies which are now forced to trade electricity through the stock exchange. Therefore, price relationships between fuels are the main driver of demand for individual fuels. This paper provides an analysis of the change in the value of the Clean Dark Spread with changes in coal and electricity prices depending on the assumed CO2 permit price. These calculations allow one to determine the theoretical maximum price of coal under certain market conditions resulting from the prices of electricity and CO2 emissions allowances. Taking into account that in the future the main competitor of coal for power generation could be natural gas, the issue of so called 'gas parity' is explained. The results of the analysis demonstrate the maximal potential level of coal prices in relation to natural gas prices so that under certain conditions, coal could remain competitive.