If your home draws from a private well, the pressure your house sees is not fixed. A pressure switch on the pump or the pressure tank turns the pump on at one pressure and off at a higher one. That range is the band, and everything downstream of the pump, including a filter, softener, or reverse osmosis unit, has to work inside it. Before you compare units by tank size or media, find out where your band sits. A unit that fits a home with a strong band can starve a home at the low end.
Read your pump settings before you read any spec sheet
Locate the pressure switch near the pump or tank and note the cut-in and cut-out values. They are often printed on the switch cover or in the well contractor's paperwork. If you cannot find them, ask the well contractor to read them and write them on your install sheet. Take the reading while the house is drawing water normally, such as during a shower with the dishwasher running. A static reading with no flow tells you much less about the band you will actually live with.
Ask for the pressure drop at your peak flow
Every treatment unit lowers pressure as water passes through its media, housing, valves, and any restriction in the line. The manufacturer should publish a pressure drop figure at a stated flow rate. Compare that flow rate to the highest flow your house really needs, which is the flow when two or three fixtures run together, not the flow at a single faucet. Ask the seller what pressure drop they expect at that flow, and whether they measured it on a system like yours or are quoting a general figure.
Check the unit's minimum inlet pressure
Some units need a minimum inlet pressure to operate correctly, and some valves will not switch or regenerate cleanly below it. Subtract the expected pressure drop from the lowest cut-out pressure you recorded. If the result falls below the unit's stated minimum, or below the pressure your fixtures need to work properly, the unit is undersized for your well no matter how good its rating looks. Write the two numbers side by side on paper before the installer arrives.
Measure before and after, at the same flow
On install day, or with a temporary gauge if the installer allows it, read the pressure upstream of the unit and downstream at the same tap while you run a normal load. Gauges at the inlet and outlet show the drop as it really happens in your plumbing. Repeat the test with a second fixture open. If the downstream reading falls sharply when the second fixture opens, the unit is taking more than its share of the band, and you should raise that with the installer before the work is signed off.
Things that change the band over time
The band is not permanent. Pressure tanks lose air over time and need checking, and a worn pressure switch can drift from its settings. A filter that collects sediment adds restriction gradually, so a unit that tested fine at startup may drop more pressure after months of service. Note the date you took your readings and repeat the checks on a schedule you set and write down. Keep the numbers with your well paperwork so the next person who works on the system has them.
Questions to ask before you sign
Ask who will set or change the pump's pressure switch if the numbers do not fit, and whether that work falls under the installer or the well contractor. Ask whether the quote includes a pressure check at your peak flow, and whether the result will be given to you in writing. Ask what happens to the warranty if the unit is run below its minimum inlet pressure. If the answers are vague, the sizing was probably based on a catalog page rather than on your well.
If you are also deciding which water problems deserve treatment in the first place, the grading criteria explain how the county ratings are built, which can help you separate a real concern from a general one.