Abstract: A power management and selection system for a class 8 tractor trailer, directs excess solar and vehicular charge capacity to an auxiliary load by measuring available charge capacity from a reefer power system including a reefer battery, solar panel and charge controller for moderating solar power to the reefer batter, and measuring available charge capacity from a cab vehicle power system including a propulsion system battery and alternator. Charge logic, in a selector configured for switching charge capacity to the auxiliary load, determines which of the reefer power system and cab vehicle power system has the most potential excess charge capacity, and directs the determined excess charge capacity to the auxiliary load, while the measured available charge capacity remains sufficient for powering the respective reefer power system or cab vehicle power system.