The size of a tankless hot water heater required for a whole house is determined by a few variables, including the size of the household, the number of bathrooms, hot water usage habits, and the desired flow rate. A tankless water heater should provide approximately 2-3 gallons of hot water per minute (GPM) for each shower or bathtub and approximately 1 GPM for each sink or appliance. A tankless water heater with a flow rate of 7-10 GPM may be sufficient for a typical household with 3-4 bathrooms. However, it is best to consult with a licensed plumber or HVAC contractor to determine the proper size and model based on the household's specific needs and conditions.