No, you can't claim the credit if the builder installs the system and pays for it, even if you "reimburse" the builder or get an itemized bill of sale.
If the builder installs the system, the builder is eligible for a different credit (that is only available to builders and not homeowners).
If you make a contract with the solar installer and you pay the solar installer, then you can claim the credit, even though the home is under construction, and not technically owned by you at the time of the installation.
