The documentation on ReadVarsEso is in the EnergyPlus Input Output Reference, search for "Using ReadVarsESO".

The ReadVarsESO program is distributed with EnergyPlus as a simple approach to converting the standard output files (eplusout.eso, eplusout.mtr) into files that can be put directly into a spreadsheet program and then used to create graphs or do other statistical operations

ReadVarsESO is launched automatically at the end your EnergyPlus run. By default, it will convert ALL THE VARIABLES you have requested (with a default limit of 255 columns, which is an old pre-2007 Excel limitation. You can override that)

Using an .rvi file allows you to customize which variables you'd like to get, and in which order, so you can a data of appropriate size for your needs.

Let me give you a concrete example, based on one of the first example file that comes with EnergyPlus I found that has an rvi file. This is "5ZoneAirCooled.idf" and its attached "5ZoneAirCooled.rvi". (As a reminder, example files are in: "C:\EnergyPlusV8-2-0\ExampleFiles" by default on Windows).

I chose this project because it's got 6 zones (5 + 1 plenum), and I modified and shortened it to make my point clearer.

Here is the class OUTPUT:VARIABLE in the IDF file after modification.

!-   ===========  ALL OBJECTS IN CLASS: OUTPUT:VARIABLE ===========

Output:Variable,*,Site Outdoor Air Drybulb Temperature,hourly;
Output:Variable,*,Zone Air Temperature,hourly;
Output:Variable,*,Zone Mean Air Dewpoint Temperature,hourly;

So here, I've got three variables, which should be returned for every appropriate object (because of the "*"). If you don't specify an .rvi file, you'll end up with a CSV file called "5ZoneUncontrolled.csv" that will have 13 columns:

• One for the Outdoor Air temperature
• 6 columns (5 zones + 1 plenum) for the zone air temperature
• 6 columns (5 zones + 1 plenum) for the zone mean air dewpoint temperature

Now, let's say I only want to return in the first column the Zone Air temperature but only for the zone "SPACE1-1" and then the outdoor air temperature, and that's it. Then I could just put an .rvi file in the same folder as my .idf file with the following lines in it, and that's what I would get.

eplusout.eso
eplusout.csv
SPACE1-1,Zone Air Temperature
Site Outdoor Air Drybulb Temperature
0

The first two lines are first the input file (where to read the output variable values) and the output file (where to store it). The 0 in the last row is to mark the end of the file.

Some specific information for JEPlus can be found in JEPlus documentation here.