The relevance of using real interest rate in HOMER. Why it is better than using normal bank rate of interest?  In a real project, the LCOE should be based on prevailing prices isn't it? If , for example, I have to calculate tariff for an electrification project should I add some % of money with each kWh sold on top of HOMER output to come up with a commercial LCOE? 

If we used the nominal rate we have to also ask for the expected inflation rate over the project life. Different factors might inflate at different rates so we use the real rate just to reduce the number of inputs

Setting tariffs is a more complicated topic that involves a lot of regulatory and policy issues. I wouldn't use LCOE as a tariff, but the modification wouldn't just be a simple %. It depends on what you want the tariff to cover; 1) just the operating cost, 2) operating and replacement costs, 3) operating, replacement, and capital costs. If you are including capital costs then how much of a return on capital do you want to allow and how do you want to handle debt payments that will be paid off after a certain number of years. What you really need to douse HOMER's capital and operating cost outputs to create a cashflow model of the commercial entity and then develop a tariff that gives the owners of that entity what they need. There would be important tax and subsidy issues that are very country-specific, which is why we don't do that inside HOMER.