buy your output at the rate you'd pay for it.
2-unreliable output
3-large initial financial commitment that is totally dependent on you producing surpas to get the savings. I feel this is often ignored in the "evals" oh, you pay $800 a month now? well, you'll only pay $200 goin forward, then somewhere in the fine print they will talk about how much your pay for financing your savings.
4- longevity? will it really last 10-20 years and still be producing at a high level?
5-maintaniance. not sure about this one.