PSI vs GPM vs CU

Pressure washers come in a variety of designs which can greatly effect their PSI, GPM and CU (Some refer to CU as ECU)
  
PSI vs GPM vs CU: First of all, let’s explain the acronyms. PSI stands for Pounds per Square Inch. This is the pressure rating of your machines. GPM stands for Gallons Per Minute, the flow rate of your pressure washer. CU stands for Cleaning Units or ECU for Effective Cleaning Units, which is PSI multiplied by GPM. All of these terms refer to the power of your pressure washer.

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To clean effectively, a machines must provide ‘agitation’ to scrub off the dirt and ‘flow’ to rinse it away. Think of the pressure (PSI) as the agitation that is applied to the surface that you are cleaning and think of the flow (GPM) as the rinsing force that carries the dirt away.

Homeowner machines tend to run between 1200 and 2700 PSI. Contractor-grade pressure washers tend to run between 3000 and 5000 PSI. More power means faster work, but more power also means more potential for surface damage. Wood decks, for example, are often cleaned at pressure as low as 300 PSI because 3000 PSI will rip the wood to shreds. Most contractors will settle for 3000 PSI because that amount of pressure is adequate for most jobs. Truth is that most contractors would prefer to have 3500 or even 4000 PSI if they could get it.

GPM is much more important to most contractors than PSI. Since most contractors use cleaning chemicals to do most of their pressure washing work (the fastest method) their job becomes one primarily of rinsing rather than washing. The cleaners do all of the cleaning, and the contractor rinses the dirt away. When you think about that method, you realize that the more flow you have, the faster the job is rinsed. Therefore, most experienced pressure washing contractors recognize that GPM is more important to them than PSI.

PSI (power) will help you break the chemical bond between the cleaning surface and the dirt. Once the bond is broken, the extra PSI does nothing to speed up the cleaning time.

The higher the GPM, however, the more surface area a machine can clean. For example, a 2000-PSI model with a 2 GPM flow rate might clean approximately 5-7 square feet per minute. If the same unit had a 3 GPM flow rate, it might clean 8-10 square feet in the same amount of time.

In this business, contractors sell “the finished job”. The contractor who gets that job done in two hours might be making $50 per hour. The guy who gets the same job done in one hour makes $100 per hour. Which one do you want to be?

Dealers of homeowner machines like to refer to CUs when they show you machines. This number is the result of multiplying the PSI by the GPM. If you have a pressure washer with 3000 PSI and 4 GPM, you have 12000 CUs. For homeowner pressure washers, this is a good comparison of the power you are buying. For professionals, CUs have little meaning. GPM is most important, and PSI is less important, and the CU formula makes them both equal. The best solution is to talk to a dealer who really understands what you are trying to pressure wash because he will steer you to the right GPM and PSI for the job.