FAQ

A BTU is the amount of cooling/heating required to lower or raise 1 pound of water, 1°F at one atmosphere. Assuming that you are near 1 atmosphere (you aren't on top of a mountain or something), 400 gallons of water is about 3,335 pounds so to cool this from 70°F to 45°F (25°F change) requires 3,335 pounds x 25°F change = 83,375 BTU. To change 400 gallons of water just 1°F requires 3,335 BTU.

You might need or want to apply a time factor to your analysis. Perhaps you want to cool 400 gallons of water from 70°F to 45°F in 2 hours. The cooling system would then need to remove only half of the 83,375 BTU's each hour so your cooling system could produce one half of the cooling it would need to cool the water in 1 hour.

Here is a handy formula for calculating the cooling requirement of water:

BTU = Gallons x Temperature Change (°F) x 8.33.