Keep these sizing requirements in mind as you switch to tankless water heaters.
When a customer called and said they were always running out of hot water, you either suggested a 40-gallon water heater with a larger burner, or a 50-gallon water heater. (For some “McMansions” you install two 50-gallon water heaters, parallel piped, even though there normally is no justification for that much hot water.)