Most of the operations that use them will tell you that the only way to keep them running is to have one employee who is charged with doing the routine maintenance work on them.  Too often they get assigned to "whoever has the time", which translates to "nobody", and one day there is no oil in the engine and then there is no electricity.  As for size, most of what I have seen is that a 900 cow dairy is right on the borderline of generating enough power in sales or usage to justify the up front investment.