I haven't seen that part of his work, but it makes good sense to me--a larger part of the cost of solar is installation costs compared to many other technologies and those costs are extremely variable. He may simply be using "installed base" because that is the best available measure that actually captures all of the costs.
I realize this is late notice, but I just found out. He goes on at 8pm and will likely be answering questions by 9:30. I would like to ask him some great questions and I will ask as many of the best that I see here as possible. Any ideas?