I would like more information about how standby power generators work?

Normally they are electric start from batteries charged by a control circuit that may include UPS power during the brief time generator takes to get up to speed. Depending on location, the generator may be powered by diesel fuel, propane, or natural gas and has a starter like a car motor. Besides detecting the loss of mains power, the included circuitry normally disconnects the mains power from the generated power line to protect power company workers and to keep from trying to supply the whole neighborhood.

When the power returns, for various reasons, it is common for the switch back to mains power to be manually controlled rather than having it happen automatically especially if short repeated dropouts are common. Unlike batteries in a UPS, normally the fuel tank can be refilled if the failure continues for hours.

Yahoo! Answers' new interface is hard to use, so I may have placed this in the wrong place:

Basically, I just want to know whether an air-cooled or liquid-cooled is better, and how long the system runs for after it kicks in during a power outage. For both the mid-sized and full-sized ones.

yes