In 2013, the IRS published a notice that addressed this issue. In short, the solar panel portion of a solar powered exhaust or ventilation fan is considered eligible for the credit, not the cost of the entire fan.
This mean that the taxpayer needs to in consultation with the contractor and/or manufacturer to make a reasonable allocation of what part of the cost of the fan (and labor to install) can be allocated only to the solar panels themselves, and not to any of the rest of the fan/exhaust/ventilation system.
In the case of the prefab solar exhaust fan that sits on the roof as a single unit, it is probably not possible to break out the cost of labor allocable to the solar panels as compared to the rest of the installation, so in my opinion, only that part of the prefab solar exhaust fan that the manufacturer allocates to the solar panel part itself would qualify for the credit.
IRS Notice 2013-70
Q-29: Is an expenditure for a solar powered exhaust fan eligible for the § 25D credit?
A-29: Only the component part of a property that actually generates electricity for the dwelling unit is eligible for the § 25D credit. If a solar panel on a fan generates electricity to power the fan for use in the dwelling unit, the cost of the panel component may be eligible for the § 25D credit if all the requirements of § 25D are met; however, the entire cost of the fan is not eligible. Additionally, § 25D(e)(1) specifically allows certain labor costs to be taken into account when calculating the credit. Under this provision, a taxpayer may take into account only the labor costs allocable to the qualifying component when calculating the credit.
See https://www.irs.gov/pub/irs-drop/n-13-70.pdf
