Sec 25D(d)(2) clearly has no requirement that the credit only applies to your primary or main residence.
(d) (2)Qualified solar electric property expenditure
The term “qualified solar electric property expenditure” means an expenditure for property which uses solar energy to generate electricity for use in a dwelling unit located in the United States and used as a residence by the taxpayer.
Also form 5695 Residential Energy Credit states that the home doesn't have to be the taxpayers main home.
Qualified solar electric property costs. Qualified solar
electric property costs are costs for property that uses solar
energy to generate electricity for use in your home located in the
United States. No costs relating to a solar panel or other
property installed as a roof (or portion thereof) will fail to qualify
solely because the property constitutes a structural component
of the structure on which it is installed. The home doesn't have to
be your main home.
