The British thermal unit, or BTU, is a unit of energy, one that is used to quantify the heating and cooling power of, water heaters, air conditioners and similar equipment.
A BTU is the quantity of heat needed to raise one pound of water by one degree Fahrenheit. More BTUs mean more heating power.
A water heater is ranked by BTU input, but really means BTUs/hr; thus a 33,000 BTU heater can heat 33,000 pounds of water by one degree in an hour; or, 330 pounds by 100 degrees.
A more common measure for water heaters is the recovery rate, which indicates the amount of gallons of water that can be heated to 100 degrees Fahrenheit in one hour.
Another common measure of capacity is simply the size. Home improvement retailer Lowe's recommends a 30- to 40-gallon heater for a two-person home, 50- to 80-gallon heater for five people or more.
BTUs are more commonly used by heating, ventilation and air conditioning (HVAC) professionals in rating equipment. Homeowners will use recovery rate or simply size.