An under-sink reverse osmosis system can produce excellent drinking water and still be a poor fit for a household. The usual problem is not whether the system treats the water. It is whether the system can make, store and deliver enough treated water when people want to use it.
Do not size the system from household population alone. Two homes with the same number of people may use very different amounts of purified water. One may fill a few glasses each day. The other may use it for coffee, cooking, pet bowls, ice and reusable bottles.
Before comparing equipment, estimate the demand at the treated-water faucet and identify when that demand occurs.
List every planned use
Write down everything that will receive reverse osmosis water. Include drinking, cooking, coffee makers, kettles, pet water and refrigerator connections. If you regularly fill large bottles, pitchers or countertop appliances, include those too.
Then decide which uses might happen close together. A household may use a modest total amount during the day but create a large morning demand by making coffee, filling bottles and preparing breakfast at the same time. That short period may matter more than the daily total.
Ask each company whether its proposed system can serve all listed uses without an unacceptable wait for the storage tank to recover.
Separate production from faucet delivery
Reverse osmosis systems are often described by a membrane production rating. That figure does not necessarily tell you how quickly water will come from the faucet.
Production is the rate at which the system makes treated water. Faucet delivery depends heavily on stored water, tank pressure, tubing, faucet design and any refrigerator line connected to the system. A membrane may replenish water steadily while the storage tank supplies the faster burst needed to fill a pot or bottle.
When comparing proposals, ask for three separate answers: how much treated water the system is expected to produce under your home conditions, how much usable water the tank holds, and how quickly water should flow from each connected outlet.
Do not use the tank label as usable capacity
A pressurized reverse osmosis tank does not normally deliver every gallon represented by its nominal size. Part of the tank contains the air charge that pushes water toward the faucet. Usable capacity changes with the tank's air pressure and the water pressure feeding the reverse osmosis system.
Ask the installer to state the expected usable draw from the proposed tank at your measured incoming pressure. Also ask how much water should remain available when both the drinking-water faucet and refrigerator are connected.
If a salesperson gives only the tank's model name or nominal capacity, request the usable amount for your installation. That is the figure you need when deciding whether one full tank can handle your busiest period.
Measure the pressure where the system will connect
Reverse osmosis production changes with feed-water conditions. A catalog rating obtained under specified conditions may not match performance under a kitchen sink.
Have the company measure the pressure at the planned connection rather than relying only on a general statement about household pressure. If pressure varies because the home uses a private well, ask how performance may change across the pump's operating range.
Also ask whether temperature and dissolved solids in the source water affect the company's production estimate. You do not need to calculate those adjustments yourself. You do need the proposal to explain which actual water conditions were used to size the system.
Account for refrigerator and ice maker demand
A refrigerator connection can change the fit of an under-sink system. The added tubing creates another delivery path, and the refrigerator may draw water while someone is using the sink faucet.
Show the installer the refrigerator location and the planned tubing route. Ask whether the proposed tank and tubing can maintain suitable delivery at that distance. Confirm whether the ice maker and dispenser can operate correctly with the pressure available after the reverse osmosis system.
If the line must travel through cabinets, walls, a basement or a crawlspace, make sure the route is included in the proposal. A system should not be selected on the assumption that the refrigerator connection is a minor add-on.
Run a practical draw test before accepting the installation
After the system has been flushed and the tank has filled completely, test it using the tasks that drove your decision. Fill the bottles, coffee maker or cooking pot you commonly use. If the refrigerator is connected, operate its dispenser during the test.
Watch for slowing flow, long refill waits, unusual pump operation and leaks at every new connection. Ask the installer to explain how long recovery should take under the measured conditions in your home.
If performance falls short, determine whether the cause is tank setup, low feed pressure, restricted tubing, a valve problem or a mismatch between demand and system capacity. Do not assume that replacing the membrane with one carrying a larger rating will correct every delivery problem.
Compare proposals with the same household demand
Give every company the same written use list so each proposal addresses the same job. Ask each one to document:
The fixtures and appliances that will receive treated water, the measured feed pressure, the expected production under your water conditions, the tank's expected usable draw, the recovery expectation after a heavy draw, and any pump or larger tubing included to support delivery.
Also ask what change the company would recommend if your household begins using more treated water. Possible answers may include additional storage, a different membrane configuration or a system designed for higher direct flow. The important point is to know whether expansion requires a small addition or replacement of the main equipment.
Choose for the busiest realistic use
The right under-sink reverse osmosis system does not need to cover an imaginary worst case. It should handle the busiest pattern your household reasonably expects without running out of stored water or making people wait excessively.
Base the decision on actual uses, measured installation conditions and expected usable storage. When a proposal connects those three items, you can judge whether the system fits your household instead of relying on a membrane label alone.