Exhaustible sources of energy means non-renewable sources of energy i.e. the sources which are limited in nature or take millions of years to form. Example- Coal and Petroleum are the two exhaustible sources of energy. This is due to the following reasons: Coal and petroleum are fossil fuels which were formed in the earth very, very slowly. The coal and petroleum which are present in the earth today have taken millions of years to form and get accumulated. So, if all the coal and petroleum present in earth get exhausted, they cannot be produced quickly in nature. We will not get any coal or petroleum in the near future.