The only thing I can think of is you have an automatic transfer switch and the electrician is saying you need shed some of the load from the whole house by putting essential circuits in a sub panel that is tied to the generator.
What I did on my house was put in a double lug meter base. One set of lugs fed my transfer switch and then to house panel, and the second set of lugs went to the disconnect for the solar. Is that an option for you?