You can not claim the 30% credit for a project started in 2019.
According to the instructions from the Internal Revenue Service, "For purposes of both credits, costs are treated as being paid when the original installation of the item is completed...(2020 Instruction for Form 5695, Residential Energy Credits)".
There is no provision for a project to start in one year and be completed in a future year and qualify for the higher credit from the earlier year.
Tax credits are incentives by the government to incentivize activities in particular periods. The 30% credit was to incentivize installations for 2019. To qualify, installations must have been completed in 2019.
Because your system wasn't completed until 2020, your credit is 26%.
