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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
ft**^{2}, as calculated from the Book "Homegrown Sundwellings,"
by Peter Van Dresser, 1977 (a great book now out of print), are:

**Phoenix, Arizona: 389 therms**
**Santa Fe, New Mexico, 1,444 therms**
**Great Falls, Montana: 1,728 therms**

**It is very interesting to compare this with the
annual "insolation" (energy from the Sun - not to confused with ins**__u__lation,
which measures resistance to heat flow) falling on the same surface area:

**Phoenix, Arizona: 12,600 therms**
**Santa Fe, New Mexico, 12,110 therms**
**Great Falls, Montana: 8,870 therms**

**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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