The "problem":

Power outages and the remaining amount of water in a simple pressure tank system. Could be nearly full, or on its last drop. Simply paralleling multiple pressure tanks will not "assure" a "known" remaining capacity on hand.

Yes a reserve float switch tank system, generator, or "bottle" water on hand are all reasonable options too. Each with their own different set of problems and limitations.

Except for water stagnation concerns (months / YEARS, unfounded?) either a "dedicated" reserve pressure tank with a back flow valve and a manual ball valve back flow bypass, or 2 tanks in series, isolated with a solenoid valve (on with pump on only) and pressure switched from the 2nd tank appears to also work.

For the 2nd scenairo above is there some plumbing fitting trickery to force a good percentage of the inflow to the primary tank as it flows down the chain to the second tank? (would eliminate stagnation concerns). Such a system also runs the risk of continuous pump operation should he solenoid valve fail to open. Is there some additional fail safe that can be added for this possibility?

A better solution, solenoid switching valve(s) allowing 2 tanks to operate in parallel, with each one alternately drained and refilled according to water usage demand.

There are a variety of 3 way zone valves available and the basic switching relay to control such. But I'm having difficulty with valve specifications. Some clearly indicate that corrosion _will_ be a problem with high dissolved O2 i.e. drinking water.

It seemed to be that there should already be an established path for such a system. I've just not been able to find it.

Any insights?