You don't say what you primary water source is. Assuming your supply is a well with a pump then you should have an expansion tank on the system to mainatin pressure without constantly running the pump now. Is ounds like you want to add a reserve tank for power outages. You need to hire a plumber or well contractor to map out this system. Seems like you would need another much larger expansion tank to give you water pressure from the storage tank. Alternately, you could build a tall water tower and use gravity to provide your pressure. Good luck.

What water crises are we talking about here? Well going dry, city not providing, power outage?

If it is well water you would need the tank higher than your house to supply water during power outage, otherwise you do need a pump/generator. If you are on city water, the city tower will give you pressure during power outage, you would just need a tank to store the water.

Your zoning committee might not allow you to put a water tower in your yard, and pressure will be a lot less than what a pump could do.

Is this going to be an open cistern? You have to know how to run those in a sanitary way.

Having been in the Army in some really crappy areas where they had to do this, I understand, but you need to understand that you'll also need to have what amounts to a miniature water plant. The water will need to be treated and stirred and such. It just can't sit there. I'd consider that first before I worried about fittings.

The water supplies comes from the municipality. We live in a small town and time and again they cut the water source because the dams are empty. Thank you for the reply so far. I got an idea from a friend to put a T split on the main water supply. Ones the water is off, I just closes the main supply valve and open the "Tank water" valve. The pressure pump from the tank then pump the water from the tank into the main line. So I only will use the pressure pump when I closed the main supply. I also intend to use the tank water on weekends so that the water stays fresh.

I am relatively certain that, legally, you can not do it in this simple way. You are not allowed to connect a tank to the municipal water supply in a way that it may drain back into their system if there is a pressure drop. You should be in the plumbing forum.

I am no plumber and I am not that familiar with the plumbing code, but you might be able to fill your tank from the top with a pipe or hose that can not under any circumstances be submerged. It is quite likely that you are required to have some sort of lockout device to prevent mixing of your tank water with the municipal supply at the outlet of your tank. A simple T is not going to do it. Assuming that the local authorities will approve a water-hoarding system at all, you might need to have a parallel distribution system in your home.

There is a valve you can install to prevent back flow. Install it upstream of the t, then it will work when using either supply.

Install a check valve after your main shut off and you will be fine. I would install the tank in-line so new water is always going through. The storage tank in theory would be similar to a water heater tank but for cold water. I would also install a pressure relief on the tank with a check valve so it allows air inside so the pump will work, but if the city turns on water the water wont go all over.

After the in-line tank I would T to the pump then connect back to the line with another T. Between the two T's on the line without the pump put in another check valve. Then all you have to do is open the relief valve and turn on the pump for pressure. Everything gets fresh water so you dont get stale water in the line anywhere.