It sounds like these are automated somehow, that the generator kicks in when it's not otherwise being powered - otherwise isn't it just as much work for the port to run around starting and stopping generators?
But an automated system sounds like it has its own problems, how does it make sure it has proper ventilation and comes on at the right time? Probably I don't want the container to start venting diesel fumes when it's deep in a stack of lego surrounded on all sides.
Reefers still get special treatment at ports because they have to be moved as quickly as possible. That includes not stacking them several layers deep horizontally so that ventilation is blocked (the gensets are very obvious from the outside). They’ll often get connected while in the port too, it’s just not a guarantee. Someone with more valuable produce will often pay for preferable treatment and a power connection to reduce risk from malfunction and running out of fuel.
It has to be automated since it’s a refrigeration unit that needs to know when to turn on the cooler. The control circuit starts up the generator if it senses no power connection at that time, usually off a standard marine lead acid battery.
But an automated system sounds like it has its own problems, how does it make sure it has proper ventilation and comes on at the right time? Probably I don't want the container to start venting diesel fumes when it's deep in a stack of lego surrounded on all sides.