Suppose a plant with two identical units burns a ton (907 kg) of lignite in one unit and a ton (907 kg) of bituminous in the other. The lignite may have as little as one-third the available heat as the bituminous. This means it takes a lot more lignite coal to produce the same energy output through the generator terminals as produced by the bituminous coal.
Even though there is 2,000 pounds (907 kg) of each fuel, the unit burning bituminous will run 2 to 3 times longer than the one burning lignite, assuming both units are putting out the same power in Megawatts.