A good approach to estimating the amount of fuel you use for heating versus hot water is to look at your monthly totals and find three or four months where no heat is required. Average those three or four months, multiply the average times 12, and subtract the total from your total annual fuel bill. If you use the same fuel for other purposes, for example cooking this approach works. If you use it for occasional heat in a garage, then you will need to break out that heat from your home heat in addition to the hot water and cooking usage. At this point it is more important that you come up with a good estimate taking into account all you know about the fuel use than worry about the precision of the estimate – a few thousand BTUs one way or the other will not make a big difference in this calculation.

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Heating Degree Days (HDD) are a measure of how cold your climate is and thus how much heating you need to maintain a comfort level. It is assumed that a home will not have to turn on the heat until the average outdoor temperature falls below 65 degrees. Thus, One HHD is when the average temperature is 64 for one day; 20 HDD is when the average temperature is 45 for one day. The daily HHD are added up for a year to get a location’s HDD. The examples given are based on national weather data collected and averaged over a 30 year period.

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