Downsizing The Main Breaker

If your main electrical panel is not suitable to handle the inverter breaker due to the limitation of the 120% rule or if your electrical panel is center-fed (100% rule may apply instead of 120%). The cheapest and simplest option to give your electrical panel more capacity is to downsize or derate the main breaker, that is, replacing the existing main breaker with one that has a lower rating, if your electric loads allow for it. 

To determine whether downsizing the main breaker is an option, you would need to first get load calculations for your panel to make sure the total loads in your panel do not require more power than the new breaker can supply, as this would result in nuisance tripping. Second, you would need to find out if a particular main breaker size is manufactured for your main electrical panel brand and model -- oftentimes it is difficult to source certain breaker sizes, especially if you have an older main panel brand that has been discontinued. 

IMPORTANT: Replacing the main breaker poses a serious risk and should only be done by a licensed electrician and also requires coordination with the utility to cut power to your electrical panel while this work is being done.