Gallons per day is the usual headline number on an RO system, and litres per hour is the same claim in different units. Both are measured under test conditions: a stated inlet pressure, a stated water temperature and a new membrane.
Three things move the real number
- Pressure. Below the rated inlet pressure the output falls away, and on a pump-free system there is nothing to make up the difference.
- Temperature. Cold water is more viscous and passes a membrane more slowly; the same system produces less in winter.
- Age. Output declines as a membrane loads, which is what a replacement interval exists to bound.
Rated output is not tap flow
A daily figure divided by twenty-four does not describe what comes out when a glass is filled. Systems with storage deliver at the rate the stored volume allows and then at production rate; systems without storage deliver at production rate throughout. Which of those suits a household or a kitchen is the subject of tank or tankless.
Per-model outputs and the minimum pressure each is rated for are published on the model pages and in the manuals. Where a figure was never published, the system finder on this site says so rather than estimating one.
