Most buying guides start from products. Start from four questions instead, and the catalogue narrows itself.
1. What is in the water you start with
A municipal supply, a well, a tank, a supply that is hard or discoloured or intermittently under a boil notice — these are different problems and they select different systems. Utilities publish an annual water-quality report; it is free and it is more useful than any product page, including this one.
2. How much filtered water you need a day
Drinking and cooking for a household is a different figure from a café's service, and both are different from a laboratory's. Published capacity is a daily figure for continuous production, so compare it against a day's use rather than against how quickly a glass fills.
3. How much room there is
Under-sink space is usually the binding constraint, and it is the one most often measured last. Systems that integrate their stages into a single cartridge body take less of it than an array of separate housings, and they leave fewer connections to seal.
4. What pressure the mains gives you
This is the question that decides whether a pump-free system is available to you at all. Membranes need pressure; a system without a booster pump takes it from the mains. Every model that states a minimum inlet pressure is telling you the condition under which its published capacity holds, and a top-floor flat at peak hour is exactly where that gets tested.
Where the water is not a municipal tap — a river, a lake, a tank fed from one — KomiBright's A95R is the model rated for it. If the four answers above are to hand, the system finder on this site narrows the range in about a minute.
