As far as durability and reliability goes, you want a unit that’s protected from moisture particles bouncing off of whatever surface you may be cleaning and one that has thermal protection so the motor doesn’t burn out from extended use.
The pressure washer hose should be long enough to maneuver around without jerking the unit mounted on the wall and also rust-proof with sturdy wand and trigger.
A common mistake that people new to pressure washer make is choosing a model simply based on the high numbers such units are rated. Believe me, getting the unit with the most power for your budget is totally fine, but there is more to this than meets the eye.
The cleaning power of a pressure washer can be measured by using several methods. You could use the motor/engine horsepower (HP), Amps, and Watt rating, or you could use the three or two most important numbers not related to these engine ratings. You should look out for the output in gallons per minute(GPM), pounds per square inch(PSI), and the combination of both GPM and PSI, cleaning power (CP) which you can get by multiplying PSI with GPM.
Cleaning power varies from a few thousand to tens of thousands. What it really means is that you’ll be able to get rid of a ten-year-old stain on your driveway if the pressure washer has a larger CP rating, but if the washer lacks proper pressure adjustment module, you’ll likely destroy fragile surfaces using the same pressure setting.
So, if you do not need hardcore stain remover, you might want to consider something in the middle or lower range.
The design and features of the pressure washer matter a lot. If the manufacturer decides to build the unit with durable, heavy-duty elements, then they’ll most likely support all of that with a sturdy frame which is pretty handy especially in a wall mount pressure washer. This frame serves as a support for holding the motor, protecting the engine, and for storing Q-Meg nozzles.