A BTU of sweet oil is fundamentally different than a BTU of sour oil. Sour oil is contaminated with sulfur and requires special refineries with higher energy costs. Once high quality reserves are depleted, however, producers will need to implement cost-effective methods for bringing greater volumes of subquality gas to the marketplace. A BTU of coal is fundamentally different than a BTU of wood. Coal contains more energy per pound than wood, which makes coal more efficient to store and transport than wood.
Solar radiation is fundamentally different than natural gas.
Natural gas is fundamentally different than oil shale, etc. Moreover, physical constraints limit how quickly energy can be extracted from a non-renewable natural resource. Hubbert's prediction proved to be remarkably accurate. Yields have risen slightly compared to Hubbert's original estimate, but the timing of the peak and the general downward trend of production were correct. Hubbert showed that oil production peaks and starts to decline when approximately half of the EUR oil has been recovered.