Tank: peak-hour demand
A tank heater is sized for the busiest hour of the day. Showers dominate: each one uses flow × minutes, and only part of it comes from the tank because hot water at 120 °F is mixed with cold to about 105 °F.
Example: three 8-minute showers at 2.0 GPM with 50 °F water: hot share = 55 ÷ 70 = 0.79; demand = 3 × 2 × 8 × 0.79 = 37.7 gal, so a 40-gallon tank. Check that its First Hour Rating (FHR) on the EnergyGuide label is at least the peak demand.
Tankless: flow and temperature rise
Example: two 2.0 GPM showers at once with 50 °F water heated to 105 °F: 4 GPM × 55 °F × 500 = 110,000 BTU/h (32 kW). Gas tankless units go up to about 199,000 BTU/h input; a 32 kW electric unit draws about 134 A at 240 V.
Frequently asked questions
What size water heater for a family of 4?
Four back-to-back 8-minute showers at 2.0 GPM need about 50 gallons from the tank, which rounds up to a 65-gallon tank. With 7-minute or staggered showers, a 50-gallon tank is enough.
How many GPM tankless do I need?
Add the flow of fixtures used at the same time: about 2 GPM per shower and 1.5 GPM per faucet or dishwasher.