Right transfer switch for connecting small generator to a 100amp service?
We don’t know your panel type but I think a few of us would suggest a 30 amp power inlet &amp; 30 amp double pole back fed breaker 10 awg wire with a mechanical interlock kit that won’t allow power to the panel without turning off the main.
(If you compare prices and the cost of a 20 amp inlet is only 10-15$ Less go with the larger one upgrading in the future would require replacing everything but the mechanical interlock itself so going a little larger now may save the need in the future)
After this you can select the circuits to energize and it is code compliant and one of the least expensive methods.
This will allow even future upgrades of the generator 2x of what you have and because the 30 amp is more common it may be cheaper I am not sure as I have always installed 30 and 50 amp 240v inlets.
Note I have suggested this on 4 small generators I have installed 2 went with my suggestion 2 ended upgrading once they realized they needed a larger generator.