The pressure tank in a well water system plays a more important role than many homeowners realize. This tank holds a supply of water for your home to draw down on so the pump itself doesn't turn off and on every time you use a tap. While you can't have a pressure tank that is too large for the system, you can definitely install one that is too small for your home's needs. Undersized pressure tanks may save you a little money on materials up front, but it'll likely cost you much more in the long run in maintenance for the well water system for the following reasons.
Excessive Pump Wear
Since the pressure tank's main job is to keep the well pump from running too often, it'll lead to a shorter lifespan for the pump if it's not properly sized. An oversized pressure tank only offers an even longer interval before the pump switches on, resulting in a longer lifespan for the equipment. Undersized tanks leave the pump short-cycling, causing it to burn out or putting wear on other components like the pressure switch.
Higher Chance of Rupture
Anything that causes the well pump to fill the pressure tank more often than a few times a day will increase the chances of rupture in the air bladder inside the tank. The air bladder expands as the tank empties to ensure there's still plenty of pressure to keep water moving into the rest of the plumbing system. If that bladder breaks, the tank will fill with water. The pump will switch on when the tank has only partially drained in this case in order to maintain water pressure, resulting in short-cycling and wear on the pump. Proper sizing ensures the pressure tank lasts for years before needing replacement due to a ruptured air bladder.
Unnecessary Electrical Costs
There's a cascading effect of plumbing problems when you choose an undersized pressure tank for a well water system. If the small tank causes the pump to turn on and off more often, you'll notice a rise in your electrical bill as well. When the air bladder in the tank ruptures, the cost of running the pump will rise even higher as it has to run every time you use your water supply. Investing in a larger pressure tank during the installation of your well may result in savings for the entire lifespan of the system in both reduced maintenance and lower electrical use.