A sensitivity variable is an input variable for which multiple values have been specified. HOMER performs a separate optimization procedure for each specified value. For an explanation of why you would do this, see Why Would I Do a Sensitivity Analysis?
It is easy to specify multiple values for an input variable. All variables for which multiple values can be specified have a sensitivity button beside them. The sensitivity button for the Annual Real Discount Rate can be seen below next to the input field:
To specify multiple values, click on the sensitivity button and enter any number of values on the sensitivity values dialog box:
The values do not have to be evenly spaced, and you can enter them in any order you want. If there are other sensitivity variables, they will be listed in the drop-down box labeled "Link with". By selecting one of these variables, you can perform a linked sensitivity.
When you click OK, the sensitivity button will display the number of values that have been specified for the Annual Real Discount Rate:
For capital, replacement and O&M cost variables, rather than performing a sensitivity analysis directly on these costs, there is a multiplier variable which scales these values. If, for example, you entered a PV Capital of $1000/kW, then Capital Cost Multipliers of 3, 4, and 5 would correspond to effective PV Capital Costs of $3000/kW, $4000/kW, and $5000/kW.
For variables specified in the component cost tables, the sensitivity concept is slightly different. Inputs like the initial capital cost are not defined by a single value, but by a column in the component cost table. There may be just one value in that column, but there may be more. For each such variable, there is a multiplier variable which is accessed by clicking on the sensitivity button below the appropriate table column. Pictured below is the wind turbine cost table and the sensitivity buttons for the initial capital cost, replacement cost, and O&M cost.
Because you can enter several values for these multiplier variables, they allow you to do a sensitivity on any column. The multiplier simply scales the whole column up or down. You can edit the first value in the sensitivity table for a multiplier variable. That's not the case with normal sensitivity values, where the first value in the sensitivity table is the value entered in the corresponding edit box.
HOMER designs an optimal hybrid system for each sensitivity case.
For more information, see Why Would I Do a Sensitivity Analysis?