A genset is, at its core, a self-contained diesel generator that powers a reefer container's refrigeration unit whenever shore power isn't available — which, on a long-haul road route, is most of the journey. Without one, a reefer is just an insulated box.
Operators generally choose between three configurations. Clip-on units attach directly to the front of the container and are easy to fit and remove, making them well suited to short stops at ports or during intermodal transfers. Underslung (or undermount) units are anchored beneath the trailer chassis instead, at a typical weight of around 700kg, and tend to suit operations where the same chassis is used across multiple container swaps. On-chassis units sit somewhere between the two, built into the trailer itself.
Fuel consumption is the number that actually drives operating cost. Most gensets burn somewhere in the range of 2 to 4 litres of diesel per hour depending on age, load and ambient temperature, with newer models commonly rated to run continuously for several days between refuels. Engine age and maintenance condition matter here as much as the unit type — an older, poorly maintained genset can burn noticeably more fuel for the same cooling output.
Emissions standards are tightening the field further. Newer units are increasingly built to meet EPA Tier IV and NRMM Stage V requirements, with manufacturers adding digital controllers that track fuel use and temperature in real time and start-stop automation that cuts idle burn. None of this is exciting on its own, but on a multi-day Mombasa run it's the difference between a cargo that arrives at spec and one that doesn't.
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