Wait, I just answered my own question. I was also wondering why it seemed that I kept building Solar and Coal Plants when I realized, "They cannot coexist!" So I would get the gold from selling them. Am I correct?
Later, as I thought about it more, it would be an interesting way of getting gold. I was running at a -20 deficit (had 3000+), so I didn't really care, as the game is wrapping up anyway. Anyway, build one, then build the other, get gold, build the other, get gold, if you have enough cities, you could pull it off.