Powered pressure washers scour areas by applying a powerful flow of pressurized water. At the outset, you have to to understand the measurement terms used when describing pressure washers. Pounds per square inch (PSI) is the way water pressure is measured. The greater the PSI, the more power the pressure washer will have. Gallons per minute (GPM) describes the amount of water hitting the surface you are cleaning. A pressure washer with a higher GPM will take less time to clean surfaces than a model with a lesser GPM. The finest pressure washer is usually categorized by its GPM, PSI and its power source whether they are gas or an electric power washer.