Tank size in gallons is only part of the story. Here is how to match a heater to your household.
Tank heaters: first-hour rating
The first-hour rating is how many gallons of hot water a heater can supply in an hour starting full. It is listed on the EnergyGuide label. For example, Rheem lists 69–85 gallons for its Performance Platinum models and A.O. Smith lists 73 gallons for a 40-gallon ProLine XE power-vent model. Pick a rating that covers your peak hour, such as back-to-back showers plus laundry.
Tankless heaters: flow at your temperature rise
Temperature rise is the difference between the incoming water and the hot water you want. Navien rates its NPE-240A2 at 11.2 gallons per minute at a 35°F rise but 5.6 at a 67°F rise. Takagi's sizing map puts Southern California incoming water at about 62–77°F, so heating to 120°F is a rise of roughly 43–58°F here, between those two numbers.
Add up simultaneous use
For tankless, add the flow of fixtures likely to run at once, such as two showers and a kitchen sink, and choose a unit that covers that flow at your rise. An installer can measure your fixtures and incoming temperature.
General information, not professional or legal advice; follow your manufacturer's manual, and your installer and local permit office have the final word. Sources:Rheem Performance Platinum powered damper; A.O. Smith ProLine XE GPVL-40; Navien NPE-2 QuickFacts (NPE2-QF001); Takagi residential brochure (4/2023). SD Water Heaters is not affiliated with any manufacturer; brand names are used only to identify products, and codes can differ by model, so confirm in your own manual.
Water Heater Replacement in San Diego →