Pressure washers also help in gardening work and can be used for removing grime, algae, and dirt. When purchasing this machine, there are some basic ratings that must be considered. First is HP that is horse power that its engine can generate. Next is Pounds per Square Inch (PSI) which calculates amount of water pressure that is between spray tip and pump. Other two are cleaning power units (CPU) and gallons per minute (GPM). Water pressure components include pumps, drive system, electric motors, motors, engine, and gas engine.