This thesis presents a thermal energy analysis of an office container located at the University Department of Professional Studies of the University of Split. The office container is used as an external laboratory for thermal energy systems at the Department of Mechanical Engineering. In the theoretical part of the thesis, the nZEB standard and the conditions that need to be fulfilled in order to meet the standard were discussed, with reference to the required energy. In the practical part of the thesis, a technical description of the office container is given. Calculation of heat energy losses and gains, as well as calculation of the ventilation system, was made. Considering the results of the calculation, a heat pump was selected for the heating and air conditioning of the office container, and a device for ventilation of the office container with thermal energy regeneration was selected. Measurements were made of the basic operating parameters of the ventilation system in the summer operating mode of the heat pump, as well as the measurement of electricity consumption during the operation of the heat pump and the ventilation system.