Sizing an ultraviolet system is one calculation and one judgement. The calculation is the flow the house can draw at once. The judgement is how much margin to leave, because everything else that goes wrong with these systems, a filming sleeve or an ageing lamp, reduces the delivered dose in the same direction as excess flow does.
Work out peak simultaneous draw
Add up what can plausibly run together: a shower, a washing machine filling, a kitchen tap, an outside tap in summer. That total, not the household's average use, is what the chamber has to treat at full dose, because dose is intensity multiplied by exposure time and exposure time falls as flow rises. Sizing on average use produces a system that treats properly at three in the afternoon and inadequately at eight in the morning, which is exactly the wrong way round.
On a well, the pump decides too
Flow through the chamber is set by what the pump delivers rather than by what anyone chooses, so check the pump's output as well as the fixtures. A large pump can push water through a small chamber faster than the household would ever draw it deliberately. Where the numbers are marginal, a flow restrictor ahead of the chamber caps the rate at the unit's rating, which is a cheap way to guarantee the dose at the cost of some peak flow.
Read the dose, not the wattage
A wattage figure describes the lamp, not the treatment. What matters is the dose delivered at a stated flow and water clarity, ideally from independent validation, and units are commonly offered in two duty classes: a higher grade for water known or suspected to carry contamination, and a lower grade as a supplementary barrier on water believed safe. A well with a positive coliform result needs the higher grade, and that choice changes the sizing as well as the price.
Leave margin for what will happen anyway
The sleeve will film, the lamp will age, and the household may add a bathroom. All three reduce delivered dose. Sizing with no margin means the system is adequate only when everything is perfect, which is briefly after each annual service. Size up rather than down, keep the pretreatment maintained, and fit an intensity sensor so the drift is visible. The EPA notes private wells are not federally regulated, so testing is the owner's proof. Our whole house UV guide covers the components.
Questions people ask about whole house uv filter
What flow should I size for?
The peak simultaneous draw of the house, not the average, since dose falls as flow rises.
Does the well pump affect sizing?
Yes. Flow through the chamber is set by the pump's delivery, which can exceed what the household would draw deliberately.
Is a higher wattage lamp better?
Wattage describes the lamp, not the treatment. Compare the validated dose at the flow you will actually run.
Should I size with margin?
Yes. Sleeve filming, lamp ageing and added fixtures all reduce delivered dose, so a system sized exactly is adequate only when new.