Quick take
A 9 GPM system can work when peak demand stays modest. If two showers plus other fixtures can push above roughly 7–9 GPM, a 15 GPM-class system gives safer treatment margin.
Why 9 GPM may be enough
Low-flow showers and staggered use can keep a three-bathroom home under the smaller system’s validated flow.
Why 15 GPM can be the better buy
A larger UV unit preserves dose when several fixtures run together and reduces the risk of exceeding the validated flow during peak use.
VIQUA reference points
The Arros family is offered in 9, 15 and 22 GPM classes at the manufacturer’s 30 mJ/cm² standard condition.
Do not ignore upstream flow restrictions
Sediment filters, iron filters and softeners can all reduce available flow. Size the UV from the actual treated-water flow reaching the chamber.