[stevehull]

With different fuels, the number of BTUs, calories or other energy units would be exactly the same to heat a specific quantity of water up by a number of specific degrees.

However, combusted fuels have different heat outputs per unit volume. Liquid fuels use a true volume (liters, gallons, etc) and gaseous fuels use a volume at a specified pressure and temperature.

What you want to calculate (I think) is the amount of a fuel, and its cost, to provide that amount of heat.

Per volume, when combusted, diesel has more heat than gasoline. Per volume, natural gas has more combusted heat than propane. Peat has less heat (per weight) than wood, but coal has a higher heat (by weight) than the other two. The costs of each is different.

It gets even more confusing when you use the power of falling water (as with dams) to describe energy as water in itself has no energy. The energy with dams is the result of stored potential energy by virtue of gravity (up hill water can release power when it flows down hill).

Energy to heat water is just that, but differing fuels have different inherent energy densities per volume. Some fuels are burned, other situations, such as in heat pumps, simply require you move energy from one state to another.

Hope this helps.

Steve