The credit can reduce your tax liability, but it can't be refunded if your tax liability is used up by other credits or deductions. For example, if you made $5000 in withholding and payments, and you owed a further $1000 when you file your return, your liability for the year was $6000. Or, if you made $5000 in payments and withholding, and get a $1000 refund, your liability for the year was $4000.
Anything that increases your income tax owed is something that the credit can be used against. That would include capital gains from the sale of property. In fact, if you plan a large solar credit, it can be a good strategy to find ways of temporarily increasing your taxable income to take advantage of the credit -- like selling assets, or converting traditional IRAs to Roth IRAs.
The only tax that can't count against the credit is self-employment tax.
