BTUs required to heat a volume of fluid in an hour.
Using the equation:
BTU = 8.345404 (lbs per gallon of water) * Specific Heat * Specific Gravity * Delta T (°F) * Gallons
For example:
Assuming you have 300 gallons in a tank and wish to know the BTU requirement
to raise it from 40°F to 120°F in an hour.
You would use 80°F for your Delta T
and 300 for your gallons.

