Typically, the rule-of-thumb is that you want a 40 gallon tank for 1 to 4 people, a 50 gallon tank for 4 to 6 people, and a 50 gallon high recovery or 75 gallon tank for 6 to 8 people. Energy consumption for 40 and 50 gallon standard water heaters is quite comparable.Click to see full answer. Keeping this in view, is 40 gallon water heater enough?A hot water heater that has an FHR within a couple gallons of the number you just calculated should meet your family’s needs. 2-3 people need a 40-gallon water heater. 3-4 people need a 40-50 gallon water heater. 5 or more people need a 50-80 gallon water heater. how much does it cost to run a 40 gallon gas water heater? Water Heater Costs by Gallon Tank Size (Gallons) Price Range* 40 $320-$1,600 50 $400-$2,200 75 $900-$3,000 80 $1,000-$3,000 Also asked, how long should a 40 gallon water heater last? about 8 to 12 years How long does a 50 gallon water heater last?So, a fully heated, 50-gallon tank will deliver roughly 33 gallons of hot water at any given time. The average shower uses 2 gallons of water per minute, so that same 50-gallon tank is good for a little less than 17 minutes of hot water.