Water Supply During Power Outages
A bladder pressure tank set at normal pressure settings is not a reliable way to store water for power outages. Murphy's law says that when the power goes off, your pressure would be at 41 PSI, when using a 40/60 switch. This means that your tank only has a few cups of water stored. There is no way to make certain that the tank is pumped up to 60 before the power goes off. You would just be lucky if the tank had anywhere near 10 gallons of water in it when the power goes off. And Murphy lives with all of us so this is probably not going to happen.
Decreasing the air pre-charge in the tank to say 20 PSI, is the only way to be sure there is any water in the tank during a power outage. This way even if the pressure was 41 when the power went off, you would still have a few gallons expressed from the tank as the pressure drops from 41 to 20 PSI. Decreasing the air charge from the 38 PSI recommended for a 40/60 switch to 20 PSI, means the 10 gallon drawdown will only be about 7 gallons between 60 and 40 PSI. But you would have 3 or 4 gallons left as the pressure drops from 41 to 20 PSI. Reducing the air charge also causes the bladder to over stretch and shortens the life of the tank. Of course the CSV eliminates so many cycles that the bladder should still last a long time.
So the conclusion is that this will work if you use less air pressure in the tank. However, the best way to have water during a power outage, is to spend the money on a generator instead of a larger tank. This will give you water for longer periods of power outage such as when the lines are down for days at a time. 10 gallons in a tank would only last a few minutes. With a generator, you have water as long as you have fuel.
When using a larger pressure tank, you don't want the CSV to take too long to refill the tank after the faucets are closed. A CSV comes preset at 50 PSI. So below 50 PSI it will let the tank fill at the full pump rate. Above 50 PSI, the CSV will only allow 1 GPM to fill the tank. So with a tank that has 10 gallons of drawdown with a 40/60 switch, the CSV set at 50 PSI would take about 5 minutes to refill the tank. We really don't want it to take longer than about 3 minutes to refill the tank. So you would need to back the pressure switch down to 35/55, or set the CSV up to 55 PSI. This way we are only topping off the last 25% of the tank at 1 GPM.