Best case IMO for these units is where fuel supply is readily available (thus the interest by O&G) and they can be run as primary, partial, and also backup. Solar is similar in this respect as it can lessen draws from the grid and provide some at least minimal backup in the event of an outage. I really can't see these as a "traditional" backup in the sense of a diesel generator for example, but very much in play with availability of NG and subsidiary benefits such as heating and cooling.