The rules have recently changed. See here. 

https://news.energysage.com/solar-tax-credit-explained/

There has never been an end to the credit carryforward just an end to the new solar credits which has now been extended for several more years.  What you can do is find a way to increase your income to use the credit faster ... converting an IRA to a ROTH is a good way. 

There is no time limit on using up the carried-forward credit. 

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@Bsch4477 wrote:

The rules have recently changed. See here. 

https://news.energysage.com/solar-tax-credit-explained/

 

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That article is a mess because it combines the Investment Tax Credit (which is only for businesses) and the section 25D credit for residential homeowners.  There are no significant changes to the residential clean energy credit other than the extension of the 30% amount.  I wrote a FAQ on it here.

https://ttlc.intuit.com/community/tax-law-changes/discussion/revisions-to-section-25d-residential-cl...