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Max PSI vs rated PSI, and why GPM counts just as much
Updated
PSI is how hard the water hits. GPM is how much water arrives to carry the dirt away. Cleaning speed depends on both, so the trade uses cleaning units: PSI times GPM. The catch is that listings often show the max of each, and those two peaks never happen together.
Cleaning units calculator
Cleaning units = PSI x GPM. Use the rated or working numbers if the listing gives them. A 15 amp, 120 volt outlet supplies 1,800 watts, which caps water power at about 4,100 cleaning units even with zero losses. Job ranges are rules of thumb, not a promise.
What we saw on real listings
- The Westinghouse ePX3500 says it in plain words: 2500 max PSI and 1.76 max GPM "at lowest pressure", then 2000 rated PSI and 1.2 rated GPM. Max times max is 4,400. Rated times rated is 2,400.
- The Sun Joe SPX3000 title says 2500 max PSI. The bullets say 2030 PSI working pressure and 1.2 GPM. That is 2,436 cleaning units.
- The Greenworks 3000 PSI listing shows 3000 PSI and 2.0 GPM, which would multiply to 6,000. But the same listing states 3,300 cleaning units. So at 3000 PSI the flow is about 1.1 GPM, by our math.
- The Karcher K1700 skips the max number. It lists 1700 PSI and 1.2 GPM and says they are CETA certified.
A quick reality check
A standard 15 amp, 120 volt outlet can supply 1,800 watts. Turn that into water power and the ceiling is about 4,100 cleaning units, and that is with a perfect motor and pump, which do not exist. So when max PSI times max GPM lands above that, as 4,400 does, you know the two numbers are not from the same moment.
How to find the rated numbers
- Read past the title into the bullet list. Look for the words rated, working or TruPressure.
- Look for PWMA or CETA. Both are trade groups with a test standard for PSI and GPM claims.
- If a listing shows only max numbers and names no certification, assume the working pressure is well below the title. No-name listings claiming 4000 or 5000 PSI from a wall plug fail the outlet check above.
What cleaning units do you need?
- Under 2,000: cars, bikes, grills, patio furniture.
- 2,000 to 2,800: decks, fences, siding, patios and short driveways. Most home electric washers, like the ePX3500, sit here.
- Over 2,800: long driveways and big concrete areas. See the Greenworks.
These ranges are rules of thumb. A lower number still does the bigger job. It just takes longer.