In a metric system of measurements, one milliliter of water takes up one cubic centimeter, weighs one gram, and one calorie of energy is needed to heat it up to one degree Celsius, which is one percent of the difference between its freezing and boiling points. The hydrogen contains one mol of atoms.
In the U.S. system of measures, the answer to the question "How much energy will be needed to boil a gallon of room temperature water?" will be "Goed to the ground", because it will not be possible to directly compare any of the values used.