I can't find it now, but I would say YES, the installation cost qualifies to be part of the credit.
The credit for solar power for a business/rental property is part of the Investment Tax Credit on Form 3468 (tax Code §48). It is a parallel of the personal tax credit in Section 25D, and I have read somewhere that the same rules for installation apply (and the installation costs are specially included for the personal credit, so they should also be included for the business/rental credit).
