PSI vs GPM: what each number controls
PSI (pounds per square inch) measures water pressure — the force behind blasting off caked-on dirt or paint. GPM (gallons per minute) measures water flow — how fast that pressure clears debris and rinses a surface. Multiply PSI by GPM to get 'cleaning units' (CU), a single useful number for comparing overall cleaning power across different machines.
Electric washers: light-duty and low-hassle
Electric models typically run 1,300-2,000 PSI at 1.2-1.8 GPM — plenty for cars, bikes, grills, patio furniture, and small decks. They're quieter, need no fuel or oil changes, start instantly, and are lighter to maneuver, but the cord limits range and they can't keep pace with gas models on large driveways or heavily soiled surfaces. Electric is the right call for occasional residential use where convenience beats raw power.
Gas washers: driveways, siding, and heavy jobs
Gas models typically run 2,800-4,000+ PSI at 2.0-4.0 GPM, cutting cleaning time significantly on large concrete surfaces, fences, and multi-story siding. They're louder, need fuel and periodic oil maintenance, and weigh more, but deliver roughly 2-3x the cleaning units of a typical electric unit. For a full driveway, a large deck, or annual house washing, gas usually earns its keep despite the added upkeep.
Matching PSI to the surface
As a general guide: 1,200-1,900 PSI for cars, motorcycles, and screens; 2,000-2,800 PSI for decks, fences, and grills; 2,900-3,300 PSI for driveways, concrete, and siding; 3,300+ PSI for heavy stripping, stubborn stains, or commercial/fleet use. Test on an inconspicuous area first and run the widest nozzle angle (25-40 degrees) that still gets the job done — narrow zero-degree tips can etch wood, damage soft siding, or strip paint at high PSI.
Nozzle tips and protecting the surface
Most pressure washers ship with color-coded tips: red (0°) is a concentrated jet for the toughest stains only, yellow (15°) for stripping and heavy-duty cleaning, green (25°) for general all-purpose work, white (40°) for delicate surfaces like cars and siding, and black (soap) for low-pressure detergent application. Start with a wider-angle tip and work down to narrower, rather than starting narrow — the safer way to avoid surface damage.
Frequently asked questions
What PSI cleans a concrete driveway?
Most driveways clean effectively at 2,900-3,300 PSI with at least 2.0 GPM. Heavily stained or oil-marked concrete benefits from the higher end of that range or a surface cleaner attachment.
Is higher PSI always better?
No — higher PSI raises the risk of etching wood, stripping paint, or damaging soft siding and vehicle finishes. Match PSI to the surface and use the widest effective nozzle angle rather than defaulting to maximum pressure.
Electric or gas for a small townhouse patio?
Electric is usually sufficient and more convenient for a small patio, deck, or car-washing routine, given the shorter cleaning runs and lack of large concrete area.
What does GPM affect that PSI doesn't?
GPM controls how quickly loosened dirt and debris rinse away and covers more surface area per minute. Two washers with identical PSI but different GPM clean a driveway at noticeably different speeds.