Payments mean nothing. You purchased the system in 2018 and it was fully operational in 2018. You claim the *entire* cost (or whatever it is you claim for this) on your 2018 tax return. Doesn't matter if it takes you 50 years to pay for it.
The credit is claimed as of the date the solar installation is "placed in service" -- installed, inspected and turned on, even if not paid for yet.
What if the system is installed, inspected and operational but is not being used as of December 31 because the agreement with the power company is not finalized?
The regulation says the property is placed in service when it is “ready and available for its specifically assigned function”. There are 5 factors the IRS looks for, although no one factor is controlling:
1. Approval of required licenses and permits;
2. Passage of control of the facility to taxpayer;
3. Completion of critical tests;
4. Synchronization to the power grid for generating electricity to produce
income; and
5. Commencement of daily or regular operation
You have some flexibility to determine what the assigned purpose is. For example, if the solar panels are generating electricity for you but are not connected to the grid yet to sell electricity back, you can decide that the main assigned purpose is to make electricity for you—then the property is in service. If the main purpose is to sell electricity to the utility, then it is not in service yet.
If the solar panels are not even making electricity for you, I would probably say they are not in service. But I am not an attorney.
Thank you! Can you tell me the regulation number that you reference?
See these links
<a rel="nofollow" target="_blank" href="https://www2.deloitte.com/content/dam/Deloitte/us/Documents/energy-resources/us-er-placed-in-service-date-issues.pdf">https://www2.deloitte.com/content/dam/Deloitte/us/Documents/energy-resources/us-er-placed-in-service-date-issues.pdf</a>
<a rel="nofollow" target="_blank" href="https://www.journalofaccountancy.com/issues/2014/apr/placed-in-service-date.html">https://www.journalofaccountancy.com/issues/2014/apr/placed-in-service-date.html</a>