Pressure washer PSI and water flow rate are the two most important factors used to determine the power capability of the tool, but they indicate different things. A pressure washer's PSI rating only indicates the amount of pressure applied to break bonds between a substance and the substrate or surface material. The GPM or flow rate more accurately summarizes the efficiency of a pressure washer. The more water that flows out in a minute, the faster the work can be done.
PSI and the Pump
The pump is designed to produce a maximum amount of pressure at a rate of flow specific to the construction of the pump. The factors that influence the rate of flow, measured in gallons per minute, are the pump speed, cylinder bore or diameter, and the stroke or length the plunger or piston has to travel within the cylinder. As water cycles through the pump and is pushed through the attached spray nozzle, pressure is affected by the size of the orifice located on the inside of a nozzle.
The maximum PSI in which different pressure washers produce varies with each machine. Electric and gas-fueled pressure washers are available, with gas-fueled units being the more powerful, thanks in part to their greater engine displacement, torque and horsepower. Light-duty residential pressure washers typically offer between 1000 and 2500 PSI. Heavy-duty consumer models may produce 3000 PSI. Heavy-duty industrial pressure washers are capable of 4000 to 5000 PSI.
The different PSI ratings of the many available pressure washers indicate what the cleaning machine is capable of. Pressure washers producing 1000 to 2000 PSI can be used to clean vehicles, gutters, fences and decks. 2000 to 3000 PSI pressure washers work to clean patios, walkways, pools, bricks and home siding. The most powerful pressure washers, the 3000 to 5000 PSI units, can effectively strip paint, remove the toughest grease spots and separate hardened concrete from steel as well as other industrial applications.