I put $32k on my roof. Made about $15k back the first year, then around $5500 each of the next two years. Should be the same for the next three years, then drop considerably as the tax breaks expire.
It's profitable, but more like a high yield bond than a stock market investment. I would have made more money by now by putting $32k in the market index in 2014. Fortunately I did that too.