Sizing a water heater starts with your own week, not with a chart on a showroom wall. The question is how much hot water your household draws, and at what hour, compared with what the unit can supply at that hour. Walk the house with a notebook before you ask anyone for a recommendation.
Map the hot water draws
Write down each use of hot water and the time it happens. Showers, the dishwasher, the washing machine, the kitchen sink, and any bathtub each have their own pattern. Note the mornings when two or more run at once. That overlap is usually what empties a tank, far more than the total used across a day.
Find your peak hour
Pick the busiest hour of an ordinary day and the busiest hour of a weekend. Those two hours set the planning input. A household that showers in a tight window before work is planning for a different load than one that spreads its use across the evening, even when both have the same number of people.
Ask the installer for the basis in writing
Ask how the proposed unit was sized and what peak draw it was matched against. Ask what happens during the peak hour you wrote down, not only on the printed rating. Ask what changes if a fixture is added later, such as a second shower or a larger tub. A recommendation without a stated basis is a guess you cannot check.
Compare stored and on-demand options by the peak hour
Some households keep a stored tank, and others use a unit that heats water as it is drawn. Each option behaves differently at peak. Ask each installer to explain what the proposed unit delivers during your busiest hour, how long the supply lasts, and how quickly it refills. Compare those answers, not the brand names.
Check the space the unit needs
Measure the spot where the unit will sit. Note its height, its width, the clearance the installer expects, the access needed for future service, and whether a drain pan and a drain path are already in place. Confirm the vent and the fuel connection with the installer before you commit, and keep the measurements with the quote.