Your tax liability is the amount the IRS keeps at the end of it all. For example, if you paid $5000 by withholding or as a payment, and get a $500 refund, your liability was $4500. This is line 22 of your form 1040. The solar credit can not be used to go against self-employment tax or penalties.
If you buy and sell securities, I assume you owe at least some capital gains tax, which is calculated on schedule D and then flows to form 1040 where it will show up on line 22 after any other taxes or credits. If you have tax on line 22, that can be reduced by the solar panel credit. Any leftover credit would be carried forward to next year.
Having a large solar tax credit would give you an opportunity to do some tax planning, such as selling stocks with large capital gains, to lock in the gains and pay no tax (by using the credit). Just be careful that you avoid the wash sale rule (don't buy back the same or similar investment for at least 31 days).
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