"Tax liability" does not mean that getting a refund disqualifies you. It means your income tax BEFORE withholding and tax credits.
For example, let's your tax return shows $10,000 of income tax, but you had $15,000 withheld from you paychecks, resulting in a refund of $5,000. In that case, the Solar Credit is applied against the $10,000.
So in that example, your $7,000 Solar Credit would reduce your 'tax liability' to $3,000. You would still have had $15,000 withheld from your paychecks, so now your refund will be $12,000 (rather than $5,000).
Does that make sense?
