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How is thermal (heat) energy measured, and how well does the Sun provide?

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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