Propane is much more efficient than natural gas. A single cubic foot of propane is rated at 2,516 BTUs, while the same amount of natural gas is rated at only 1,030 BTUs. This means that a fireplace that uses propane burns only half of what a natural gas fire will burn in an hour, though the propane burns much hotter. Though propane is usually a bit more expensive to buy, it burns cleaner and more efficiently than natural gas will, so it is a much more economical gas to use.