You would not need to turn anything off when not using water. The pressure switch will shut the pump off when no water is being used and will turn it on automatically when water is used. The little bit of extra electricity is because the CSV will keep the pump running for as long as the shower is on. Without a CSV the pump would cycling on and off while the shower is on, which is not good for the pump, but uses slightly less energy.
Pressure tanks only hold about 25% of there volume as water, the rest is air. So even an 80 gallon pressure tank only holds about 20 gallons of water. A 4.5 gallon tank only holds 1.2 gallons of water.
You will have 400 gallons in the storage tank when the city supply stops, but you cannot count on the pressure tank storing any water. An 80 gallon pressure tank would only have 20 gallons in it if the pressure happen to be at 60 when the power went off. But you have no way of making sure the pressure will be at 60 before the power or city water supply shuts off. It is luck of the draw, literally. If you have used 19 gallons of water around the house and the pressure is at 41 when the power goes off, there is only 1 gallon of water left in an 80 gallon tank. And Murphy's law says it will always be at 41 when the power goes off.
Having a faucet on the bottom of the 400 gallon storage tank would allow you to gravity flow some water to buckets when there is no power. If you want water with pressure, you would need a generator to keep the boost pump working.
You would still want a 400 gallon or larger storage tank to draw from. A float valve of electric solenoid with a float switch in the tank would keep it full as long as the city is supplying water. Similar to the following picture.
