They are "Pressure" tanks,so the pump doesn't run continuously!
I have a small one (20 gallons) at my house.When there is electricity,the waterpump cycles on and off every few minutes when you are using water.When you are not using water,the tank maintains pressure in the system,the waterpump should not turn on and off at all.If it does,you have a leak somewhere.Sometimes my pump cycles on and off almost continuously when using water.It is then time to disconnect the pressure tank,and let all the water out,so it will work again.If you use the water and there is no electricity,the pressure in the tank will give you a decreasing amount of water for a few minutes.If you have a "Tinaco",the tinaco will give you water at the rate the "gravity feed" your system allows.The higher your tinaco in relation to your "point-of-use" the greater the pressure.We have great pressure on the first floor,"not-so great" on the second floor!
It is always a great idea to have "redundency" in your "infrastructure" in the DR.That means a "Backup" for everything.We have water from the "street",most of the time.That goes into a big undergrount cistern of about 5000 gallons.When the pressure is "low" in the street,we have a "Bomba Ladrona",or a "Robber Pump" that "sucks" water into the "Cisterna". A "Big" waterpump takes water from the cistern,and presurizes the household water system,and keeps the tinaco filled at the same time.It ain't easy being "Civilized" in the "Jungle"!!
"Oh!",I forgot,we have a water well for when the street is dry!