Tank heaters: think in first-hour rating
The number that matters for a tank is its first-hour rating, the amount of hot water it can deliver in a busy hour. It is printed on the yellow energy label. A rough guide is about 12 to 15 gallons of first-hour rating per person in the house, more if several showers run back to back in the morning.
Tankless heaters: think in flow
A tankless unit is sized by how many gallons per minute it can heat at once. Count the fixtures that run at the same time on your busiest morning. A shower uses about 2 gallons per minute, a kitchen faucet about 1.5, and a dishwasher or washer about 1 to 2.
- One shower plus a sink: about 3.5 gpm
- Two showers: about 4 gpm
- Two showers plus a dishwasher: about 5.5 gpm
Upstate winters matter
Groundwater in the Upstate is much colder in January than in July. A tankless heater has to raise winter water by about 70 degrees to reach a comfortable shower temperature, which lowers the flow it can deliver. Size for the coldest month.
Fuel and space
Gas tanks recover faster than electric tanks of the same size, so electric homes often go one size up. Heat pump water heaters need room and air around them. Tankless gas units usually need a larger gas line and new venting.
Try the calculator
Our water heater size calculator takes your household size, bathrooms and fuel type and gives a starting recommendation for a tank size and a tankless flow rate.
Questions readers ask
Is a bigger water heater always better?
No. An oversized tank wastes energy keeping water hot that you never use. Size for your real peak demand.
How many gallons does a family of four need?
Usually a 50 gallon gas tank or a 50 to 65 gallon electric tank, depending on how many showers run back to back.
Sidney Treadway edits Homeowner Help for Plumbcraft Services, turning the questions our plumbers hear on calls into plain, practical articles for Upstate homeowners. About the editor