AC Panel Back feed ?

If you have a 200 Amp breaker... If you have a 150 Amp breaker, then the back feed breakers can be 40 Amps (120% rule) + (200 amp rated panel-150 physical main breaker) amps = 90 Amp solar back feed.

This article is pretty useful:

https://homeinspectioninsider.com/breakers-electrical-panel/

Things have changed over time... But if this is a new(ish) panel, they may list the number of circuits supported.

The total amperage of breakers (circuits) can add up more than the panel/main breaker can support. The assumption (for residential) that not all circuits will run at maximum rated current (electric water heater, electric stove, electric dryer, HVAC system, etc... The rest (outlets, lightning, etc.) probably do not.

For commercial installations, there is a chance that the circuits can operate at rated current (heaters, kilns, ovens, heavy machinery, etc.)--So adding up the actual continuous rated loads is critical.

Many (most? all?) panels can have "double tapped breakers" that allow you to install 2x more branch circuits vs the standard single tapped breakers.

You can run into other issues... For example if using aluminum (copper clad aluminum) wiring, you have to step up in size AWG (14 AWG copper => 12 AWG CuAl wiring).

-Bill