@ocelotjoy
I need to refer you to someone with more experience with landlord-tenant situations.
@Carl
@Hal_Al
In general, if you are renting at below market rate, that prevents you from taking full advantage of your rental situation. For example, suppose fair market rent is $1000 per month, and your expenses for mortgage, utilities, insurance and repairs are $800/month. You can also deduct depreciation (wear and tear). Suppose that is $300/month. In that case, even though you have a positive cash flow of $200/month, you have a tax loss of $100/month that can offset other taxable income. However, if you are renting at below market rate (say $600 per month) the most expenses you can deduct are $600. Meaning that you have no taxable profit, but you lose the benefit of those remaining expenses that can't be deducted.
I believe that days rented at below market rate are counted as personal use. However, I would like to have that double-checked. You are still required to report the income, but usually your expenses will be more than the rent and that will negate the income but not result in further deductions, as noted above. Here are some links.
https://www.irs.gov/taxtopics/tc415
https://laporte.com/knowledgecenter/tax-services/tax-consequences-of-charging-below-market-rent
https://www.irs.gov/publications/p527
Turbotax should do this for you if you indicate you are renting below market rate.
Then the language used in the solar credit section 25D is "non-business" use. Here, my conclusion is that "personal" use as defined in the rental regulations is the same as "non-business" use as defined in the solar credit, so you could take the full credit. However, you may want to run this past your own tax professional.
