Water heaters are chosen by First Hour Rating (FHR)—how many gallons of hot water they can deliver in an hour of peak use—not only by tank gallons. Energy.gov style worksheets add showers, baths, sinks, dishwasher, and laundry in the busiest hour.
Shower gallons = showers × minutes × gallons per minute. A 2.0 gpm head for 8 minutes is 16 gallons. Tubs are often counted near 20 gallons. Sinks, dishwashers (~6 gal), and washers (~20 gal) add on.
Match a tank whose yellow EnergyGuide FHR meets that total, or a tankless unit that can hold the peak GPM at your incoming groundwater temperature.
Backup power for a well pump or the same house is the solar battery calculator. Whole-house cooling is the AC ton calculator, and a bathroom floor after the tank is set may need the tile calculator.
How this calculation works
FHR ≈ sum of peak-hour fixture gallons. Tank range follows common 30–80 gallon residential bands.