The tax on capital gains is not a separate tax. It's included in the total income tax on your tax return.
A tax credit does not offset income. It offsets the tax on your tax return. But a credit does not offset any particular part of the tax. The total credits are offset against the total income tax. The Residential Clean Energy Credit, which is the credit you can claim for solar panels, is a nonrefundable credit. So your total nonrefundable credits, including the Residential Clean Energy Credit, will be offset against your total income tax, including the tax on your capital gains.
