[GWN7]

It all depends on volume, not preassure. Most citys water supplys are about 65 PSI (it can varry from 50 PSI to 75 PSI depending on where you are accessing it) and the volume of water you get out of the city supply depends on the size of your water line.

What is the volume of your power washer? You will have to look up what it's GPM (Gallons per minute) flow rate is. With power washers they run between 1.5 GPM to 3 GPM on average. So if your power washer is 3000 PSI and uses 3GPM your addition of a 20 gallon storage tank would give you about 7 minutes usage of your power washer at full volume (not factoring in the refill rate of water supply to the garage).

If you do put the storage tank in your garage you should install a water supply back check valve so nothing can go back into they water supply system. While they are not required for a home they are for a business (up here that is).