December is always high in solar because of the ITC. I would stay away from solar unless you really know how to underwrite it.
That high revenue in December makes me think that they cut some corners to get an install date of last year.

You should ask them if they do split pay with their lenders. If they do split pay, then the revenue amount will be about 20-30% higher each month.
If this is in California, then they are going to go belly up this year