No, the solar credit for your personal home is not able to be sold (although the business credit could have been eligible for that).
The unused credit will continue to carry forward until you use it up.
While there are many factors involved, some people that have pre-tax retirement accounts, such Traditional IRAs, have converted them to a Roth account. That is a taxable transaction creating more tax, but the solar credit would be used against that. But as I said, there are many factors involved as to if that is in your best interest or not.
