The solar rebate for 2020 is 26%. Can I claim a portion of that rebate for 2020, and the balance in 2021?

You enter the entire thing on the tax return for the year your installation became operational.  You can't choose to delay any portion of the credit...BUT...for any of the credit that is unused for your 2020 taxes, the remaining portion will carryover to be used the next year.   The tax software automatically uses any other non-refundable/non-carry-forward credits first and will calculate the carryover $$ amount.... IF appropriate..  The Solar Credits are non-refundable, but the unused portion one year, does carry-forward to the next year if not used .  How far this will continue to be the case ???...but apparently does go thru all of 2021.