Federal Tax Credit (if I buy materials?)

[foo1bar]

Thanks, I see the difference. What about if a system is purchased and put into service in a particular year, and the tax credit is taken, then in the following year the system is expanded and more components are added to that system that was installed the previous year. Can an additional tax credit be taken for the new items only in the year of purchase for those items even though it was an addition to the existing system?

I believe that an "expansion" years later would be considered a new installation. And as a new installation it is also eligible to be used for the federal credit.
I probably wouldn't try to push it so far that a tax court might consider the two installations to really be one being artificially and illegitimately split. Tax courts have ruled in other cases to consider "two" transactions to really be one. But frankly you're unlikely to be audited over something like that. And it wouldn't make that much difference in the total tax anyhow, so they probably wouldn't try to argue about it. But IMO I'd just take the approach of "If a dozen people were looking at what I'm doing from the outside - would they come to the same conclusion as I have?"