[JRTJH]

A 6 gallon water heater will provide 6 gallons of hot water.

If you use the electric element (1100 watts at 120VAC) you'll add about 4 gallons the first hour, for a total of around 10 gallons of hot water the first hour.

If you use the gas heating system you'll add about 8 gallons of hot water the first hour for around 14 gallons the first hour.

If you use both the gas and electric heating systems, you'll add about 12 gallons of hot water the first hour for a total of around 18 gallons. Quite a bit more using both. In fact, with a "water saving showerhead" it's usually enough to actually take a "warm" (not hot) shower and rinse off with warm (not cold) water.

When you consider that a "water saving showerhead" flows about 2 gallons per minute, if you mix 50% hot with 50% cold, you'll be using a gallon of hot water per minute. It doesn't take long for the initial 6 gallons to be used, usually just long enough to get your hair shampooed and partially rinsed before you're shivering. Take advantage of all the BTU's of heat you can put into the water heater or you'll be cold faster than you deserve.