Why would the tank not be full at all times?

The pump fills up the tank then shuts off when it reaches full pressure.

At that point, the pressurized tank supplies water to the house.

When the tank pressure drops to the minimum cutoff, the pump turns on and
refills the tank.

If the power goes out when the tank is full, we have 80 gallons of water.
If the power goes out when the tank is about the reach the minimum cutoff,
we may only have 5-10 gallons.

I'd figure out a way to always keep it full.

That's kind of like ensuring the gas tank in your car is always full.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com