For many two-bathroom homes, a validated 9 GPM-class system is a practical starting point. VIQUA Arros 9 is rated 9 GPM at its 30 mJ/cm² standard condition and 7 GPM at 40 mJ/cm².
Size from peak flow, not household population
Two showers, a faucet and an appliance can create more demand than headcount suggests. Use realistic simultaneous fixtures to estimate peak GPM.
Dose changes the rated flow
The same UV chamber can have different validated flow rates at different dose targets. VIQUA publishes 9 GPM at 30 mJ/cm² and 7 GPM at 40 mJ/cm² for Arros 9.
Prefiltration matters
CDC notes UV works better when water is filtered first. Sediment and turbidity can interfere with UV delivery, so well-water pretreatment should be sized before the UV unit.
Leave flow margin
If calculated household peak flow is close to the system limit, choose the next size up rather than running continuously at the edge of the validated rating.