The exceptions would be in cases where the well is somewhat marginal and isn't capable of producing much water before experiencing drawdown. Drawdown occurs when water is being withdrawn from the well more quickly than than the well is being recharged by water entering the bottom of the well pipe. A tip-off this situation may be present is if the well has only a one-half or three-quarter horsepower pump and a fairly small tank, or if neighbors indicate there are well problems whenever a drought occurs. A camparitively small pump and tank can be an indication the well driller new the well was somewhat marginal and didn't want to create drawdown by installing too large a pump or tank.

Conversely, if you have a one horsepower or larger pump and a 42 gallon or larger equivilent tank, there is probably no reason not to go with a larger tank.

A larger tank provides more water reserve under pressure, resulting in more even pressure under use, the pump cycles on and off less frequently, the tank fills and empties less often. This benefits your electric bill, and is easier on the tank, pump and pressure switch. A larger tank can handle multiple water demands with less pressure reduction than will a smaller tank.

For a two bathroom home we recommend at least a 120 gallon equivilent tank, for a three bathroom or larger home, we recommend a 180 gallon equivilent tank.  Of course the number of people living in the home can certainly decrease or increase that recommendation. If a single person is living alone in a house, a smaller tank such as the H2Pro WWT35 or the WR120 would certainly be adequate for them and an occasional guest.

The bottom line on tank size is what you as a customer want in performance. If you don't mind when taking a shower having the water pressure vary and don't have other residents making water demands while you are in the shower, a smaller tank will suffice.

If you are a person that demands even pressure in the shower while someone may be running a dishwasher in the kitchen, and while someone else flushes a toilet in another bathroom, you definitely need a larger tank - a 120 gallon equivilent or larger.