The pressure washer specs that matter most

After spending up to $90 a day renting a pressure washer to clean your deck, garage, or siding, you may be tempted to buy your own. If you're a first-time buyer, you can easily run afoul of two opposing specs that describe the machine's capability and output. That's what the outdoor power equipment experts at Consumer Reports have been seeing as we unpack more than 20 pressure washers that we've just purchased for testing.

By definition, a pressure washer uses a gasoline engine or electric motor to propel water from a hose through a narrow nozzle, boosting the pressure up to 60 times. The amount of pressure is measured in pounds per square inch or PSI. The amount of water delivered is measured in gallons per minute or GPM. To get the best performance from your pressure washer, these numbers have to work together. As a rule of thumb, the narrower the opening in the nozzle, the higher the PSI but the lower the GPM. A nozzle with a larger opening delivers more water but at lower pressure.

The box of one Briggs & Stratton model we bought claims it delivers 3,000 PSI and 5.0 GPM, which would be very powerful indeed. But in truth, you get one or the other, not both. Opt for the 3,000 PSI, and you'll get the lowest GPM, 2.3. Opt for the most water output, and the PSI will drop to a mere 400. The lower figures are also displayed on the box but in much smaller type.

How to choose. Look for a pressure washer with the highest GPM at the PSI settings you need for what you'll be cleaning. The pressure needed to clean a deck, for example, is roughly 1,500 PSI. For quickly cleaning moss off an asphalt driveway, you'll want as much as 2,000 to 3,000 PSI. If you're power washing a car, which can potentially damage the paint, you'll want a lower PSI and higher water flow so you can place more distance between the sprayer and the car. Most pressure washers have adjustable nozzles.

We're just getting started with our pressure washer tests and will be posting updates on our progress. We're testing a mix of gas and electric models that come from such brands as Craftsman, Greenworks, Homelite, Karcher, Ryobi, and Troy-Bilt, as well as Briggs & Stratton. To learn more, read "Pros and cons of gas and electric pressure washers."

—Ed Perratore (@EdPerratore on Twitter)