Water heater size

Notes

• Buy first hour rating, not gallons. FHR is what the heater actually delivers in the busiest hour: the water already hot plus whatever it can reheat while you use it. Two 50-gallon heaters can differ by 30% in FHR because the burners differ. This household needs about 97 gal, which a 75 gallon gas tank covers.
• The peak hour decides the size, not the daily total. Nobody runs out of hot water at 2 pm. The number that matters is 4 showers plus the kitchen inside one morning — and a heater sized on daily average will fail that hour every single day.
• Tankless is sized in GPM at a temperature rise, and the rise is where climate bites. Incoming water in the middle US runs about 55°F, so reaching 105°F at the tap is a 50°F rise. Delivering 4 GPM across that takes roughly 100,000 BTU/hr. The same unit that manages four showers in Florida manages two in Minnesota — the box did not change, the water did.
• Electric tanks recover far slower than gas. A gas tank refills its 75 gallons in well under an hour; a standard electric one takes roughly twice as long. If the house is electric, either go a size up or look at a heat-pump water heater, which is slower still on recovery but far cheaper to run.
• Turning the thermostat above 120°F to stretch a small tank works and costs you scald risk — the reason mixing valves exist. Sizing the heater properly is the cheaper fix.