Solar/Tax question

[Grandpa Redneck]

I know we have a few solar gurus and some tax pros on this forum, so I figured this was the place to hopefully get straight answers.
Last summer grandma and I put a small solar power set up on our off grid cabin. 1680 watts, and we spent about $4500. I know there is a solar tax rebate, but I was told it only applies to your full time residence, and only if it is a grid tied system, and professionally installed. Ours is off grid, and only a weekend get away, and we did it ourselves. I was also have been told it would apply to our set up.
My questions are.
1) Which is correct?
2) is it worth it to itemize our taxes to claim the rebate if our little off grid getaway is eligible? we always have filed EZ since we don't have enough itemized deductions to add up.
3) If eligible can we claim what we spent last year on this coming years taxes? (already filed and received our return for 2020)
Thanks in advance for the help
Grandpa Redneck