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The basic unit for thermal energy in home heating applications is the "therm", which is defined to be 100,000 BTU's:
1 therm = 100,000 BTUs
BTU stands for British Thermal Unit and is the amount of energy required to raise the temperature of 1 lb of water one degree Fahrenheit. Intuitively, you can think of a BTU as approximately equivalent to the heat given off by burning one match head.
A BTU is equivalent to1055 Joules, and from this you can calculate that a therm is about 105,500,000, or 105.5 million Joules!
To get a feeling for how much energy a therm is, a home furnace is typically rated at somewhere around one therm per hour.
Typical annual heating loads, for a house with 1,800 ft2, as calculated from the Book "Homegrown Sundwellings," by Peter Van Dresser, 1977 (a great book now out of print), are:
It is very interesting to compare this with the annual "insolation" (energy from the Sun - not to confused with insulation, which measures resistance to heat flow) falling on the same surface area:
It can be seen that there is abundant solar energy for heating homes, even in Montana. Thus, we see that a home in Santa Fe should be designed to capture about 10% of the available insolation, and the homes in Phoenix and Montana should capture less and more, respectively. The art to building a home to capture just the right amount of this energy without overheating is called passive solar design.
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