Recovery of gas-fired water heaters is a function of the BTU input and overall efficiency. 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.
The BTU input can be found on the manufacture’s ID plate on the side of the water heater. The ID plate is not the big yellow sticker that shows the energy efficiency, but is a smaller plate that shows the gallons, serial number and BTUs.