Factor (EF): A
measure of water heater overall efficiency, is the ratio of useful
energy output from the water heater to the total amount of energy
delivered to the water heater.
Energy Factor (SEF): The
energy delivered by the total system divided by the electrical or gas
energy put into the system.
Rating (FHR): An
estimate of the maximum volume of hot water in gallons that a storage
water heater can supply within an hour that begins with the water
heater fully heated.
per Minute (GPM): The
amount of gallons per minute of hot water that can be supplied by an
instantaneous water heater while maintaining a nominal temperature
rise of 77°F during steady state operation.
information has been copied from the EnergyStar website. The full
page can be seen here.