Revit seems to know the elevation of light fixtures with respect to the floor below as a System Parameter. The "Offset" and "Elevation" System Parameters are not targetable by Tags or usable in Schedules for elements of the MEP Categories. This has been a Revit limitation for a while but there are a couple of workarounds to consider. 

  1. Create a New Schedule (Analyze --> Schedule/Quantities --> Lighting Fixtures) 
  2. Add the desired fields for the location summary schedule (Type, Description, Level, and Count). The Elevation field is not available and you will need to create a custom "Mounting Height" Project parameter in order for it to be available in schedules, which will require manual entry for all elements.  
  3. There is a faster way to tie the "Elevation" System Parameter to the created "Mounting Height" Project Parameter without having to manually enter it for each element. A simple Dynamo script can read all of the System Elevations of all your lighting fixture instances and write it back into your schedule. Download the "Lighting_Fixture_Mounting_Height.dym" script (link below), open it on your Revit project, and simply hit "Run" in Dynamo for your schedule to update.