When a household sizes a treatment system, the usual question is how many people live there. A house that fills a pool, runs a large irrigation zone, or has several fixtures open at once creates a different problem. The system has to keep up during the peak, and a unit sized only for daily averages can fall behind on those days.
Separate the average from the peak
Daily use tells you how much water passes through over a day. Peak demand tells you how fast it has to move at one moment. Filling a pool through a treated line asks for a sustained flow for hours, which is a different load from a shower and a dishwasher running together. Ask each provider which flow rate they sized for and how they measured it.
Gather what the sizing needs
Write down the pool volume and how long a fill takes at your hose or line. List each irrigation zone and whether any of them could run at the same time. Note bathrooms, appliances, and outdoor taps that can draw at once. Bring this list to every quote so the providers are working from the same facts.
Decide what the system treats
Many households fill a pool or water a lawn from an outdoor tap that sits outside the treated line. That is a choice, not an oversight. Ask whether the plan routes treated water to outdoor taps, whether that adds parts or pipework, and whether you are paying to treat water you will never drink. Decide that first, because it changes the size.
Ask how the unit behaves during a big draw
Ask whether the regeneration cycle pauses or runs during a long fill, and what happens to output if the unit is still regenerating when a second demand starts. A provider who answers in terms of your fill time and your zones is working from your house. One who answers with only a headcount is not.
Our methodology page describes how we handle sizing questions when we compare providers.