When your’re choosing a fuel source for your home heating system, start with available fuels in your region, because not all fuels are available everywhere and some are cleaner and more efficient than others. Based on your chosen fuel—natural gas, fuel oil, light propane gas, and kerosene are the most common—you can figure out how much heat a gallon will give you and how efficient that fuel is likely to be. Called the standard heat value, the amount of heat a fuel can produce per gallon or cubic foot is measured and reported in Btu (British thermal units). The higher the Btu produced, the more efficiently the fuel burns and the greater its heat value. Fuel oil, for example, has a heat value of 135,000 Btu/gal., while liquid propane gas produces 91,000 Btu/gal. Next you need to plug that information into the furnace selected. How many Btu/hour the furnace or boiler releases determines its output and will help to decide how much furnace you need to heat your space to the desired temperature. This is how the pros size and evaluate your heating needs. Following these formulas, you can plot out your energy usage, optimal furnace size, and desired output, too.