Deductions & credits

Correct, you can only apply 75% of the cost for the credit.

You list the entire solar system as an asset for your home (a home improvement), using as your basis, the cost minus any federal, state or local credits and rebates.  Then you depreciate the solar system like you would depreciate any other improvement that affects the whole house (i.e. 25%), as you would if you replaced the HVAC system or similar whole house improvement.  And you recapture the depreciation when you sell the house.