carried forward: https://www.taxaudit.com/tax-audit-blog/how-to-write-off-solar-panels-for-a-sole-proprietor-business

The credit first offsets income tax on the 1040 (subject to some limits, including some AMT rules).

If the credit is not used, the General Business Credit is usually carried BACK one year, then forward.  I had stuck in my mind there were some different rules for the carryback for business energy credit, but I would need to look that up.

As a side note, it may not be a 30% credit; it could be higher or lower.  The rules for the business credit are much more complex now.

How many years forward can the credit be claimed?

After the carryback, I think it is 20 years.

A quick Google search seems to indicate the special-modified carryback for the business energy credit is 3 years.