A reverse osmosis unit under the sink is a product you will live with for years, and the spec sheet hides most of what matters in daily use. Compare the units on the points below, write each one on the same line, and only then look at the price.

Start with the waste water

A reverse osmosis unit sends part of the incoming water to the drain as it works. Ask each seller for the stated ratio of treated water to waste water, and ask whether that ratio holds at your home's pressure or only at the test pressure the maker used. A unit that drains a lot needs a clear, permanent drain path. If your home runs on a well, the water used to make treated water also comes off the pump's run time, so include that in your comparison.

Check the pressure floor

Every unit lists a minimum inlet pressure. Ask what happens below that number: does the unit simply make less water, shut off, or need a booster pump? Measure your own pressure at the cold tap before you talk to anyone, because a seller's estimate is not a reading. If a unit calls for a booster, the pump, the power outlet, and the drain route all become part of the decision.

Compare the storage and the faucet

Ask for the tank capacity in writing and whether the unit uses a standard tank or a sealed proprietary one. Ask how many fittings the unit needs and whether the faucet needs its own hole in the sink or mounts on the sink deck. A faucet that needs a second hole is an extra cost and possibly a sink change. Confirm the tank's pressure setting is listed in the manual, since you will need it later.

Read the cartridge schedule and the replacement source

Ask for the full list of filter stages, the replacement interval the manufacturer states for each one, and whether the cartridges are standard sizes sold by more than one company. If a unit uses a cartridge only one company sells, you are tied to that company's prices and availability. Get the expected yearly replacement cost from the seller in writing.

Check the certification, not the claim

A product sheet that says a unit reduces a contaminant means little until you find which standard it was certified to. Many reverse osmosis units are certified to NSF/ANSI 58, and the listing for a certified unit states the contaminants it was tested to reduce. Ask the seller for the listing number, look it up on the certifier's public listing, and confirm the model number matches exactly. A contaminant that is not on the listed claim is not one the unit has been shown to reduce, whatever the brochure says.

Put every unit on one sheet

Write the drain ratio, the minimum pressure, the tank capacity, the number of stages, the cartridge schedule, the certification and listing number, and the warranty length and exclusions in one table, with a column for each unit. Blank cells are answers you still need to get. To see how we weigh details like these when we grade products, read the How Grades Work page.