Two houses of the same size can need very different treatment equipment. What matters is how many people use water, when they use it, and how often several fixtures run at the same time. Before you compare models or estimates, describe your household's real pattern. Then you can check whether a recommendation actually fits your home.
Map a normal weekday morning
Write down the sequence of water use from the first alarm to the time the house goes quiet. Note who showers, when, and whether two people shower back to back. Include the kitchen, the coffee maker, the dishwasher, and any laundry that runs early. Do the same for a typical weekend, because weekend patterns are often quite different. The goal is to find the busiest window, not the average day.
Count the fixtures that run together
Look for overlap. A shower running while the dishwasher fills and a washing machine drains is a much heavier load than the same fixtures used one at a time. Households with teenagers, frequent guests, or a home office that runs a second bathroom often hit this overlap regularly. Mark the moments when two or more fixtures are likely in use, and count them. That list is more useful to an estimator than a generic number of bedrooms.
Notice periods of low use and high use
Some homes sit nearly empty for long stretches, such as when a family travels for weeks or when one person works away from home. Others are full all day. Ask how the proposed equipment behaves after a long idle period and after a sudden burst of use. A system that performs well in a steady routine may need a different setup if the house alternates between quiet and crowded.
Decide which water needs treatment
Some households only need treated water at the kitchen tap, while others want it throughout the house. Write down where you drink water, cook with it, and fill pets' bowls, and where you simply use water for cleaning or bathing. Whole-house treatment and point-of-use treatment answer different questions, and mixing them up leads to paying for coverage you do not need or missing the place that matters most.
Bring the list to the estimator
Give the company your peak-hour map, the overlap count, and the kitchen and drinking-water locations. Then ask them to explain, in plain terms, the assumptions behind the recommendation. Ask what usage they assumed, whether they accounted for overlapping fixtures, and what would change the answer if your routine changed. A good estimator will be glad to walk through the logic with you. If they cannot explain it, you have learned something useful about the proposal.