You see, cars are not built to be as hard as concrete - the kind of surfaces that a proper pressure washer is built for. Car pressure washers are typically built to deliver low pressure levels that won't damage your paintwork.
But even with such a pressure washer that is built to supply controlled pressure for use on a car, you can't just use it anywhere on a car. You are supposed to use it on the metal parts - the bodywork, the engine and so on.
The 1500 pounds per square inch that car pressure washers are capable of delivering (which would be approximately 100 bar, to express it in another common unit), could blow a hole in the plastic parts of your car or just blow your windshield wiper right off.
Before you set out to buy a pressure washer that's built for cars, you want to try to make up your mind the exact kind of use you will put it to.
They usually sell models for different kinds of purposes. The lightweight models are best for the occasional car cleaning project at home. The electric motor won't be a heavy-duty one, and itwon't stand up to continuous use. A lightweight model is not the kind of thing for instance that they would use in a car garage where they would need to clean cars all the time.
For home use, to simply buy a lightweight unit would serve your purpose and it would save you a lot of money.
Now pressure washers that are used to clean driveways and other such tough surfaces, do deliver a high level of pressure. But they do so with very little water. Since you're using low-pressure on your car, you want to make up for it with a system that delivers a lot of water at that pressure. Typically, you should look for a unit that delivers about 2 gallons of water a minute. The higher you go, the better.
If you're trying to choose between a no-name brand and an expensive European import like Bosch, you have to understand that you probably won't notice any difference in performance at first. Both units will probably work exactly the same for a couple of years. It's the area of durability that the expensive models will do a good job in.