Which year may I use form 5695 for solar panel purchase and installation?

Only in the year they are installed.

You could deduct any prep work or labor costs that occurred in 2016 it can be counted as the cost of investment. The IRS has not provided further guidance in the instructions to complete form 5695. Therefore, the labor under the solar panels could be counted. but since the Solar Panels were not actually installed until 2017 you would file them under 2017.