Definition. A BTU is the quantity of heat needed to raise one pound of water by one degree Fahrenheit. More BTUs mean more heating power.Click to see full answer. Thereof, what is the BTU rating of a water heater?Input ranges from about 32,000 on a 30-gallon unit to 88,000 on a 100-gallon tank. A common input is 34,000 BTUs on a 40-gallon tank and 36,000 BTUs on a 50-gallon tank. The higher the BTU input and efficiency, the faster the recovery. Electric water heaters typically have one 5,500-watt or two 4,500-watt elements.Subsequently, question is, what is a good first hour rating for a water heater? Example: The water heater is 40 gallons, elements are 5500 watts (electric water heater) 40(gallons) x . 7(70 percent of the tank capacity) + 22.5(GPH at 80 degree rise in temp) = 50.5 gallons first hour rating. Answer. If your tank capacity is: 70% of your tank capacity is: 120 gallons 84 gallons Considering this, what does the BTU mean on a hot water heater? Input – The amount of gas or electricity used per hour to heat water in the tank. Higher input means the water heater can heat more water faster when needed. Natural or propane gas input is expressed in BTU’s (British Thermal Units) per hour, and State gas inputs range from 30,000 to 75,100 BTU’s.Is a short or tall water heater better?Short water heaters vary between 30 to 49 inches and hold up to 50 gallons of water. The nickname for tall water heaters is “short boy.” Tall water heaters range from 50 to 76 inches and can hold up to 100 gallons of water. They’re ideal for basements or garages where height isn’t an issue.