Stock market is not guaranteed to gain 7% over the next 20 years, but you can effectively guarantee your electricity usage is probably going to stay the same or

Stock market is not guaranteed to gain 7% over the next 20 years, but you can effectively guarantee your electricity usage is probably going to stay the same or go up and can somewhat guarantee the performance of the panels (given the warranties). It also is a useful form of diversification. If you only had $15k, it might not be the best overall investment, but if you have more money than that lying around it probably isn't a bad place to invest some of your money assuming you plan on using lots of electricity over the next 20 years. At some point having all your money in the stock market isn't the best money allocation either.