The IRS will always apply any tax refund to a previous tax debt before paying anything to you.
The solar tax credit is non-refundable, it can reduce your tax, but not more than your tax liability. Normally, if your tax liability was $5000 and you paid $10,000 into the system by payments or withholding, you would get a $5000 refund. If you apply for the solar credit in an amount of $5000 or more, the solar credit will reduce your tax liability (possibly down to zero) and you would get the entire $10,000 you paid into the system as your refund. The maximum solar credit is 24% of your cost, but you will never get more than your tax liability. So if you installed a $50,000 solar system with a possible credit of $12,000, but your tax liability is $5,000, your credit will be $5,000.
Also note the solar credit is not applied against self-employment tax, only income tax.
Then, in your case, if you are expecting a $10,000 refund but you have a payment plan from a past tax debt, the IRS will apply the $10,000 toward your past due amount. It won't recalculate the amount of future payments you owe but it will reduce the number of payments you have to make. If the past tax debt is paid off, you will get whatever is left.
