Different Solar + Storage models return identical payback calculations

I am modeling a basic solar + storage scenario in SAM and am having trouble with the payback results. I have two different models (attached) with vastly different financing that are showing exactly the same payback calculations. Everything between these models are identical besides the loan rate (base model with 4.99%, test model with 0%). Theoretically, shouldn't these models have different payback results?

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.

Thanks Paul and apologies for the bad attachments. I gained a better idea on what was happening thanks to this note in the Help section: "SAM's payback period calculations do not include debt because of the ambiguity introduced if the payback period is less than the debt repayment period: In that case the payback period might suggest that the project is "paid off" before the owner's debt payments are complete. For example, if a project with a 25-year analysis period and 15-year debt period has a payback period of 12 years, the payback period suggests that the project has paid for itself in Year 12 even though debt payments continue for another three years."