An under-sink reverse osmosis system can produce enough water over a full day and still disappoint you at the exact moment you need it. The usual problem is not total production. It is the amount of treated water available before the storage tank needs time to refill.
Choosing a tank by its advertised capacity alone can be misleading. A pressurized tank does not normally deliver every gallon named on its label. Part of its internal space is occupied by air, and water delivery changes with incoming pressure, tank pressure and the pressure required by connected fixtures. Ask the installer for the tank's expected usable draw under conditions similar to those in your home.
Start with the busiest period, not the daily total
List the treated-water uses that may happen close together. Common examples include filling several bottles in the morning, making coffee, preparing a large pot of food and drawing water through a refrigerator dispenser. Estimate how much water those tasks require before the system has a meaningful chance to refill the tank.
This peak-demand estimate is more useful than simply counting household members. Two households of the same size may use an under-sink system very differently. One may fill a glass occasionally. Another may send treated water to an ice maker, use it for cooking and fill containers for work or school.
Give the water treatment company your actual use list and ask, "How much treated water can this proposed tank deliver in one continuous draw?" If the answer is only the nominal tank size, ask for the usable amount instead.
Check how quickly the system replaces used water
The storage tank covers short bursts of demand. The membrane's production rate determines how quickly the supply returns. That rate can be affected by incoming water pressure, water temperature, membrane condition and the amount of pressure already inside the tank.
Ask how long the proposed system is expected to take to replace the amount your household typically uses during its busiest period. Also ask whether the quoted production is a laboratory rating or an estimate based on conditions measured at your house.
If your household has repeated demand periods with little time between them, a larger tank may help. A higher-output system may be the better answer when fast recovery matters. In some homes, both changes are worth comparing. Request separate proposals so you can see what each change is intended to solve.
Measure the installation space before selecting a tank
Open the cabinet and measure usable width, depth and height. Do not treat the cabinet's outside dimensions as available space. Account for the sink bowl, drain piping, garbage disposal, shutoff valves, electrical outlets, shelf supports and items that must remain stored there.
The tank also needs room for its valve and tubing. The installer should be able to remove filters, disconnect the tank and inspect for leaks without dismantling the cabinet. Ask whether the tank must stand upright or may be placed in another approved orientation.
If the tank will not fit comfortably under the sink, discuss a nearby cabinet, basement or utility area. A remote location may require a longer water line and could affect delivery at the faucet. Have the company explain how it will route and protect that line, especially where it passes through finished cabinets, walls or floors.
Include every fixture the tank will serve
Confirm whether the system will supply only a dedicated faucet or also a refrigerator, ice maker, coffee appliance or secondary sink. Additional destinations increase demand and may require longer tubing runs.
Refrigerators can be especially important because their inlet requirements and internal filters vary. Ask whether the reverse osmosis system will provide adequate pressure at the refrigerator, not just at the tank. If the refrigerator retains its own filter, confirm whether that filter should remain in place and whether it creates an unnecessary restriction.
Put the intended connections in the written proposal. This prevents a tank from being selected for one faucet when the installation is later expected to serve several outlets.
Ask about tank pressure and maintenance
A pressurized storage tank contains an air charge that helps push water to the faucet. An incorrect air charge can reduce usable volume or leave water trapped in the tank. Ask what empty-tank pressure the manufacturer specifies, whether the installer will verify it and how often it should be checked.
Also ask how the tank will be sanitized, how its condition will be evaluated and what signs would indicate a failed internal bladder. A tank that is difficult to reach is more likely to be neglected, so access should be part of the sizing and placement decision.
Compare proposals with the same questions
For each proposed system, record the nominal tank capacity, expected usable draw, estimated recovery time, required installation footprint and every fixture it will serve. Then ask what happens during your household's busiest realistic use period.
A useful proposal should explain whether the limiting factor is stored volume, production rate, incoming pressure, delivery distance or available space. If a company recommends a larger tank, ask which specific demand it addresses. If it recommends a higher-output membrane or delivery pump, ask what measured condition makes that equipment necessary.
The right fit is a system that can supply the water your household needs during its busiest period, recover before the next one and remain accessible for service. Tank size matters, but only when it is considered alongside real usable volume, refill speed, pressure, connected fixtures and the physical space in your home.