how to measure btu on a hot water storage tank
It takes one Btu per pound of water to raise the temperature by one degree Fahrenheit. The standard size of American hot water heaters is 40 gallons, and each gallon of water weighs 8.33 pounds. Calculate the Btu. If you have a 40-gallon water heater, that is 40 multiplied by 8.33, or 333.2 pounds of water. So you need 333.2 Btu to raise the water 1 degree Fahrenheit. However, most areas need more than one degree of heat added to the water to make it warm enough for showering or washing clothes or dishes.