It might be assuming no energy price increases. Given that energy e.g. increased in many german cities from 30ct/kWh to > 60ct/kWh, it might break even much faster.
On the other hand it might however also not include maintenance costs.
On the other hand it might however also not include maintenance costs.
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.