You claim the credit as of the date the property is placed in service, that means installed, inspected, and turned on. If the skylights are placed in service in a different year than the main PV system, you would claim the credit as of the date each separate device or system is placed in service.
There is no requirement in the law to place the different parts in service in the same year, and if you install one device or system in one year and another device or system in the next year, you can claim the credits in both years.
If by solar skylight, you mean skylight that generates a small amount of solar power to open and close itself for ventilation, but is not connected to the power grid and does not provide power to the rest of the house, I am dubious that it actually qualifies for any credit. If I remember correctly, I researched this a while ago and did not find an absolute written requirement that the solar panel must be connected to the grid, which would suggest that solar skylights are qualified. However, I would not take my own word for it because I am not your paid professional, and I would not take the word of the installer who is not a tax professional either. You may want to consult with a tax professional in your area.
