I'm not sure what that means. But, you can probably do both, if the house was converted from your residence to a rental unit, after the improvement was made.
You claim the energy credit on your 2015 return, on your residence. Even it the house was a rental unit, at the time you did the work, the energy credit is still claimed on form 5695 and line 53 of form 1040 , not schedule E
When you converted the unit to rental, you add the $15K to your cost basis and depreciate it over 27.5 years.