Reverse osmosis systems under sink all do the same thing, so the choice comes down to four specifications that vendors present unevenly: how the treated water is stored, how fast the membrane produces it, what happens to the water after the membrane, and what the cartridges will cost you every year. Stage counts, the number most prominently advertised, matter least.
Stage counts are mostly marketing
A system advertised with a high stage count is usually adding carbon polishing cartridges after the membrane, not adding treatment capability. The stages that do the work are constant across almost every unit: a sediment prefilter, a carbon prefilter that protects the membrane from chlorine, and the membrane itself. Everything after that is taste polishing or optional mineral addition. When you compare two systems, look at what each stage is made of and how long it is rated to last, not at how many housings are bolted to the manifold. More housings also means more cartridges to buy.
Tank or tankless
A tank system stores treated water in a small pressurized vessel so it is ready when you open the faucet, and it needs no electricity. The trade-offs are cabinet space, a bladder that eventually loses its charge, and a tank that has to be sanitized occasionally. A tankless system uses a booster pump to make water on demand, freeing the floor of the cabinet and eliminating stored water entirely, but it needs a power outlet, it makes noise while running, and if the pump fails you have no water from that faucet at all. Households with tight cabinets and a nearby socket usually prefer tankless.
Membrane output and the drain ratio
Membranes are rated in gallons per day at a reference pressure and temperature, and your kitchen will not match the reference. Cold winter feed water and modest house pressure both cut real output substantially, so a generously rated membrane is worth having if you cook a lot or fill bottles. The other number to ask about is how much water goes to drain per unit of treated water. Higher-output modern systems generally waste less, and a permeate pump on a tank system improves both output and drain ratio by relieving the back pressure the storage tank puts on the membrane.
Remineralization, taste and certification
Reverse osmosis water is low in dissolved minerals and tastes flat to many people, so some systems add an alkaline or mineral cartridge after the membrane. That is a taste decision rather than a health one, and it adds a cartridge to the annual bill. Certification matters more: look for the specific contaminant reduction claims you actually need rather than a general statement that the system is tested. Finally, price the replacement cartridges for a full year before you buy, because a cheap system on a proprietary cartridge can cost more over its life. Our best under-sink reverse osmosis guide does that comparison.
Questions people ask about reverse osmosis systems under sink
Is a higher stage count better?
Not by itself. Extra stages are usually post-membrane polishing cartridges. Compare the media in each stage and the annual replacement cost instead.
Does a tankless RO system waste less water?
Generally yes, because there is no storage tank back pressure working against the membrane. A permeate pump achieves a similar improvement on a tank system.
Why does my RO water taste flat?
The membrane removes the dissolved minerals that give water its taste. A remineralization cartridge after the membrane puts a controlled amount back.
How long does an under-sink RO membrane last?
Typically a few years if the carbon prefilter is changed on time, since chlorine reaching the membrane is what usually kills it early.