I am a former Onan/Cummins tech and am partial to Onan/Cummins products. I serviced all makes but by far the best were Onan/cummins units. Generacs I found were prone to failure. Kohlers are OK but not as relaible as Onan. The 13 Kw 3600 Rpm Onan uses a very dependable Robin Subaru engine as said earlier with an oil cooler. The 20 Kw air cooled 3600 Rpm Onan uses a Briggs also with an oil cooler. For the occasional outages they are excellent choices. For extended outage Liquid cooled 1800 Rpm is the way to go. The Onans are more expensive but quality is never cheap. They are also equipped with isochronous digital governors for excellent power quality, which is very important. Also most critical infrastructure projects I have seen use Onan/Cummins for good reason. I have seen 30 to 50 year old Onans running perfectly.