Hello,
There are a couple of ways to model a grant. You could use the Investment Based Incentive input on the Incentives page to enter it as a percentage of the total installed cost, or you could reduce the installation cost on the System Costs page by the amount of the grant.
One way to use a different payback calculation method than SAM's is to export the cash flow from SAM to excel by clicking Send to Excel with Equations on the Cash flow tab of the Results page. Then you can manually change the initial investment amount or make other adjustments in the payback cash flow to recalculate the payback period.
Without seeing your .sam file, it is hard for me to know what is causing the payback period to be higher than you expect. SAM calculates the payback period based on initial investment, electricity bill savings, and annual expenses, so adjusting one of those factors should change the payback period.
Best regards,
Paul.