How do you know that I should add some random variability on the primary load data?  Any indicator?

We recommended that you add random variability because in almost every case, electrical loads do display random variability.  In rare cases where the electrical load comes from some industrial process that is very stable, the load may be the same every day.  But in most cases, where the electrical load is the sum of the electrical demands of many people using electrical appliances and lights, the aggregate load is not the same every day.  People don’t turn on and off their appliances and lights at exactly the same time every day, so when you add individual loads together, you tend to see some average daily profile, with some amount of random variability from hour to hour and from day to day.

You can assume the replacement cost is the same as the capital cost.  That’s what HOMER does by default.  But you can make them different if you want to assume that not all of the components will need to be replaced, or that costs will rise or fall over time.  Please look up ‘Replacement Cost’ in the index of the Help system for more information.