Why must more subbituminous coal be burned than bituminous coal to produce the same energy?

Subbituminous coal must be burned in greater quantities than bituminous coal to produce the same amount of energy because it possesses a significantly lower calorific value.1

The calorific value represents the chemical energy stored in coal that is released as thermal energy during combustion. This value is directly linked to the coal's rank, which is determined by the degree of coalification it has undergone. Generally, as coal rank decreases, its moisture content increases. Because moisture adds weight without contributing to energy output and reduces the overall calorific value, lower-rank coals like subbituminous coal are less energy-dense than higher-rank bituminous coal. Consequently, to achieve an equivalent energy yield, a larger mass of subbituminous coal is required.2341