The water heater does not pressurize the house. We don't have a tank for our water heater.

A pressure tank is what's used to keep the house under pressure. That is unless you have an external tank and it's 40 or 50ft higher than the house, then you could just gravity feed the pressure to your house.

The larger the tank (or more) the more water you have on-hand if power goes out because they remain pressurized.

Available water draw is also dependent upon your pressure on the pressure tank.

Google "Well pressure tank" and "How pressure tanks work", to read the full info on them.

Depending how deep the well is you can add a hand pump (200ft) ours is 400+ft so we can't. If power goes out we have water that is available in the pressure tank and that's it.

If electric is out for along time we CAN hookup a generator to the well and get more water.

We are going to add a 2600g external tank for watering the garden, and fire control, and in the future another 2600g to feed the house. This however requires another pump because then the well will fill the 2600g tank and turn off with a float switch and the pump in the tank will then pressurize the house's pressure tank(s). The large/external tanks outside are not to be pressurized.

Essentially once we add our second tank and pump and plumb it (of coarse check valves too in their needed places) we'll have 5200g available for fire control, watering the garden, and 2600g to the house (or the full 5200 depending how I plumb it). With this setup we'd only need to run the pump in the one tank (not the ground) to re-pressurize the tank for the house. These pumps are normally a lot power hungry which is GREAT!

After we get the external / large storage tanks we`ll add another pressure tank (120g, but not that much available for draw down. There is a chart that comes with the tanks that tell you how much water you have available at what pressures).

The 2600g external tank is $999 local or $799 if I drove 2 hours. The pressure tanks are at Lowes and are $499 I believe for the 120g model but I think with our pressure we only get maybe 50g of water from it. So, cost-wise the external tanks are much more of a deal, but do "less".. or "more" depending how you look at it.

I hope that explains it more.