how long it takes to freeze up a load solid depends on the outside temperatures and how directly the cooled air can contact the frames. if it is 80 degrees F., it takes a lot longer than if it is 40 degrees F. outside. if the load is in shrink wraped boxes it takes a lot longer. at say 55 degrees out side and the load is only half what will fit in the trailer it should be frozen solid in a day or so. so maybee25 gallons at $4 would be $100 at most. the load air temperature will be down to frozen food range [10 degrees ] in less time, a few hours, the unit will then idle and save some fuel. yes it costs money to run a refer.