The credit can't be more than your income tax liability. If you have not entered all your taxable income (maybe you don't have your W-2s yet), the credit will be recalculated when your income is updated.
Your income tax liability is the tax you owe to the IRS, regardless of withholding or refunds. For example, if you paid $4000 in withholding and expect a $1000 refund, your tax liability would be $3000. Likewise, if you paid $4000 in withholding and owe a further $1000 payment, your liability would be $5000. The solar credit can't be more than your tax liability, but if you update your income or tax, the credit will also be updated.
