Real Discount Rate, Nominal Discount Rate

Hello everyone.
This is Hamza.

I am doing Economical Analysis in two software. SAM and RET Screen software. 
I have financial parameters let suppose,
Inflation = 8
Discount Rate = 10
So why nominal discount rate (NDR )is calculated in SAM? what is the purpose of NDR when i have inflation and real discount rate.

And when I input the same values in RET Screen software i get different results (NPV, payback period) why? even though other parameters/costs everything is same in both software. 

Sam uses real discount rate or nominal discount rate for calculations? 
What values I should use in both software so that i may get same NPV and PP.?

Please reply as soon as possible. I am doing a research work and I am confused what to do. 

Hi Hamza,

See the explanation for real and nominal discount rate on the Financial Parameters Help topic, for example, here is a link to the topic for the Single Owner financial model (the explanation is the same for all of the financial models): samrepo.nrelcloud.org/help/fin_single_owner.htm

In SAM, the NPV calculation uses the nominal discount rate as described in the Net Present Value topic in Help: samrepo.nrelcloud.org/help/mtf_npv.htm

SAM's financial model calculations are based on Short W et al, 1995. Manual for the Economic Evaluation of Energy Efficiency and Renewable Energy Technologies. National Renewable Energy Laboratory. NREL/TP-462-5173. www.nrel.gov/docs/legosti/old/5173.pdf

Best regards,
Paul.