Powered pressure washers clean areas by applying a mighty flow of pressurized water. First, you need to understand the measurement terms used when describing pressure washers. Pounds per square inch (PSI) is the way water pressure is measured. The higher the PSI, the more energy the pressure washer will have. Gallons per minute (GPM) describes the quantity of water hitting the surface you are cleaning. A pressure washer with a greater GPM will take a reduced amount of time to cleanse surfaces than a model with a lower 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.