Solar Tax Credit Question - Can I include solar in tax for 2021?

[Drake Hamilton]

Hello, I am deciding to go solar primarily for the federal tax credit and have a question:

If I sign all the paperwork to get solar installed NOW before the year ends, but it isn't actually installed and running until early 2022, when will I be able to write it off as a tax credit? For 2021 or 2022 taxes? 

I've heard conflicting answers from "once you get the permitting started" to "after it is completed installed and running".

Thanks!

[Jeff Shorter]

I have seen a couple references to using the business tax credit for rental property solar, has anyone done this or know about it?