In some NEM rules, the customer is not credited for excess kWh production at the full retail rate of electricity. Instead, utilities only credit the customer based on a percentage of the retail rate - maybe 5% less, maybe 40% less, depending on the utility. In many cases this is a first-come, first-served tier-based system - the 1st tier will get 100% of retail rate, the 2nd tier will get 95%, and so on until the utility reaches a defined MW capacity of distributed generation on their grid.
In order to model a percentage reduction, create a financing and select NEM (Utility Rate). Change the Export Rule to Percentage Export Penalty. Enter the reduction into the box on the right - this is the amount that will be removed from the retail rate for compensation. For example, if your utility uses 90% of the retail rate to compensate excess solar production, you should enter a 10% penalty.