I've read on two different online sources that add-on of solar panels to an existing system are not eligible for the tax credit. However, both sources are very soft in its positions. Note in the source with attached link, under "A Note on Incentives", it reads "Unfortunately, most homeowners will not be eligible for solar incentives when expanding their systems. For example, they probably cannot claim another federal tax credit. It is critical to be upfront with customers about this, so they understand the incentives before making a decision to expand their solar system. "
https://www.greenlancer.com/post/how-to-add-solar-panels-to-existing-system
