The ability to financially qualify for sponsorship will be based on his proof of non-taxable income which he will have to provide as supporting documentation....not what is entered in boxes 24 a-c on the I-864. Like I said, a formal letter is fine as an explanation of taxes not filed also.
I would write "N/A" Then, indicate the reason beside the boxes. If you were not required to file for those years, you can write "Reportable Income below reporting threshold" beside the boxes if that was the reason.
I was surprised to hear him say that there are so many residential solar installations done without batteries. I've always thought of Solar's main purpose to be a backup system with a benefit of selling excess back to the electric company. Of course, it's a different ball game when you're off-grid.