Hi Evan,
The Excel files you attached are filled with zeros. Please attach the .sam file you used to generated the Excel cash flow files if you would like me to help troubleshoot.
The loan interest rate may or may not affect the payback period, depending on other assumptions. You should be able to see why the loan interest rate is not affecting the payback period by tracing through the formulas in the Excel cash flow files.
Best regards,
Paul.