As a general rule, it is the generator supplier who must provide advice on which model is best suited. A rule of thumb is that a generator should deliver 3 times the power of the pressure washer's power.
Additional info:
A generator can be built in many different ways, and some have many safety systems and deliver accurate power, while others vary a lot along the way and typically have few protection features against misuse. Some generators are intended for electronics that require precise current and voltage, while others are suitable for tools that load the generator differently during use.
With this answer, we try to illuminate some issues so that you are at least better equipped to make a decision.
Example 1: Master P80, a large and powerful pressure washer
This machine draws 2,800 watts. A generator that delivers a maximum power of 2,800 watts is too small because generator manufacturers usually distinguish between maximum power and nominal power. In this case, we can use Honda EU30si as an example: this generator is rated at 3,000 watts but has a nominal power (continuous power) of 2,800 watts. Thus, we can proceed and say that all is well so far. The next challenge arises when we read the user manual. In the manual, we can read that maximum power should not be used for more than 30 minutes. This means that the house wash, which usually constitutes a whole morning of spraying, must be done in many rounds, which can be difficult to keep track of.
Further, we can read in the manual that mountain folk must be aware of the following: the carburetor must be adjusted when used in the mountains and even with a generator set for use 1,200 meters above sea level, one must expect a power loss of 3.5% per 300 meters above sea level. Now the generator was too small for everyone who had thought of going to mountain areas.
For those who are not going to the mountains and still hold on to hope, the next thing to read is this: the total power should not be exceeded, and what happens when you press the gun on a pressure washer? The engine starts and draws 2-3 times as much at the start moment than otherwise. If you, like many others, spray a little, stop, and spray again, the load including these frequent startups can quickly make you exceed the maximum power. In the example above, we used an expensive and well-regarded generator from a renowned manufacturer. This protects against misuse and tolerates short-term overloads relatively well.
Example 2: A small pressure washer, for example, a Go P40
This machine draws 1,700 watts and in this example, we use an inexpensive generator of 2,000 watts of unknown brand. The issues mentioned above still apply but on many cheap generators, there are big challenges with the startup. When the machine's power consumption increases at the start, the generator chokes, like a lawn mower usually does when mowing too tall grass. The engine cannot handle enough load and dies out. There may also be a fuse that trips or even worse; the voltage drops so much that the pressure washer disconnects or in the worst case breaks down. Here there is thus a greater risk that the generator, the pressure washer, or both get damaged.
Summary:
A good, old rule of thumb when it comes to pressure washers and generators is that one should count 3 times the power consumption of the generator in relation to the pressure washer's power. If you adhere to this, you should be safe that both the washer and generator will perform well. The disadvantage of this rule is that it is expensive and if there is no need for such a powerful generator for other tasks, it becomes disproportionately expensive. The alternative to following the rule is to talk to the generator dealer, tell them which model you will use and what is the expected use. A skilled retail store or perhaps the manufacturer themselves will be able to provide a qualified recommendation that satisfies you, the generator, and the pressure washer.