It's not line 18. Look at line 22. So as long as Line 22 is more than $7500 (before applying the credit), you can use the whole credit. It will reduce your income tax (but only to zero). If your non-refundable credits reduce your tax to 0 you will get back all the withholding and payments and any Refundable Credits. Unless you owe for something else like self employment tax or the 10% Early Withdrawal Penalty on IRA withdrawals.
It's not whether you get a refund or tax due. And the withholding doesn't matter. It's if you have a tax liability on your income.
