Typically, the majority of foam cannons will require a PSI of between 1,000 and 3,000 PSI to be able to operate as they are supposed to. The number increases depending on whether you want a foam cannon that is light or heavy-duty.
Most light and medium-duty foam cannons will require a PSI of between 1,400 and 2,500 and anything between 2,500 and 3,000 PSI would be classed as heavy-duty.
Different foam cannons will require different PSIs and there is no correct answer for them all, so you must double-check the requirements for the specific foam cannon that you are considering purchasing.
That way, you can align the PSI needs of your foam cannon with the power washer specifications to ensure that your foam cannon will be able to complete its job correctly.