Backfeed ?

I apologize if this ? has been asked before.
I have 320 amp service with 2 200 amp disconnects each feeding a separate 200 amp breaker panel.
I am trying to design a 10.1 kWh system for my outbuilding that has one of the 200 amp breaker panels.
My question is can I derate the main breaker in the building to 175 amp without changing the size of the disconnect breaker feeding it.
I am in central Ohio, the system would be south facing on the roof of a post frame building. I am split between microinverters and l or a single 7.7 sma string. The system would be grid tie and the goal would be to offset my energy use as close to 100% as possible. I plan on installing the system by myself. Any and all help is appreciated.

If I am understanding your question, you want to put a 175 Amp circuit breaker down stream of your 200 Amp one?

The answer is probably subject to interpretation by the inspector. What is the math that requires you to derate that breaker panel? I have 40 Amps of solar going into a 200 Amp subpanel. There are solar ready panels that have 225 Amp busbars.

Yes. 175 amp breaker downstream of the 200 amp disconnect.

I believe I need a 60 amp breaker for that size of system and my understanding is that I can only put a 40 amp breaker in the 200 panel

Electrically (Technically) you can put a smaller main breaker down stream of a bigger one and it will work fine. As Ampster said, if you hire an inspector - they may have some strange rules in your area that says you cant. But there is no danger to putting a smaller breaker down stream. You will simply be limiting everything to 175 Amps. As for what will fit in your panel - that is brand and panel style specific. You'll have to look up your panel and find out what fits.

Yes, you can downsize the panel main breaker, to allow the 60a solar. So that you don't overload the busbar. (I'm assuming that is the reasoning) as long as you don't change the wire feeding it, the 200A disconnect is still protecting the wire correctly.