When I get my 12kw or 15kw it will be an old school Detroit Diesel 2-71 based. < 5k$ rebuilt...
Someone I know has the 25kw version and it eats .75g/hr @ half power... I suspect the 12kw to eat the same since the generator head is just different and the motor is the same. .75g/hr is fine for me... WAY better than Gas, Propane or Natural Gas and the engine will last at least 15x longer than Gas, Propane or NG.
Seems like a no brain`r for me... assuming you have the room (They're large), and can enclose it away from the house (I can) to keep the sound minimal.
I don't need 12kw or 15kw but 15kw is only a couple hundred more than the 12kw, and I may need it to do some repairs (welder) in case of emergency
-Todd