The Oil Drum

"The average American household spends 20 percent or more of its energy bill on hot water, and much of what's paid for is heat lost through the thin walls of the storage tank in the basement or utility room." ...

"Most solar heating units act as preheaters for conventional units. Although the installation costs are high, owners save from 50 to 85 percent annually on their utility bill, according to the U.S. Department of Energy."

I'm definitely a fan of solar hot water systems.  A lot of fossil fuel could potentially be displaced.  I tried cruching some numbers on what I assume are reasonable numbers for a pre-heating system but for some reason it doesn't seem right.  I know (and the quote above also suggests the same) that they are more effective than the numbers I got.

Assuming the pre-heater raises the temperature of the water to 90F from 55F. (32.2C - 12.8C)  Delta 19.4 C

1gal X (3.785Liter/1gal) X (1000g/1L) X (19.4C) X (4.185 Joule/gram X degC) = 307300 Joules (per gallon)

or 291 BTU (per gallon)

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and these were some numbers I crunched of a storage system I saw at a house I visited(he used this system for both hot water and to heat his house):

Assume 150*F to 80*F useable range (delta 70*F or ~39*C)

5000gal X (3.785Liter/1gal)X(1000g/1L) X 39degC X (4.185Joule/gram * degC) = 3089581950 Joules

3089581950 Joule (.00094978 BTU/Joule) = 2934423 BTU

Or... ~23.66 gallons of gasoline (based on BTU's) of storage if the tank is at 150 degrees F.