Improved control of the auxiliary units is a possible method to save fuel in heavy vehicles. Today, the auxiliaries are often mechanically driven by the engine, and are constrained to rotate with a fixed ratio to the engine speed. This mechanical constraint results in energy losses. In the paper, the benefits of driving the auxiliaries with electricity are evaluated. The output of an electrically driven auxiliary can at every time instant be controlled to match the actual need. Considered auxiliaries are electrical generator, water pump, cooling fan, air compressor, air conditioning compressor, oil pump and power steering pump. Upper limits on what fuel saving that can be achieved if the auxiliaries are redesigned are first discussed. Simulations indicate that the fuel consumption caused by the auxiliary units is in the range of 5-7% of the total consumption. A Modelica library used for simulation of the energy consumption of the auxiliary units is presented. The cooling system of the library is described in some detail. A case study on optimal control of the cooling system is then performed. Control actuators are the electrical generator, the cooling fan and the water pump. The fan and the pump are supposed to be electrically driven. The control design is based on a simplified model derived from physical principles. It is evaluated through simulations with external variables collected from experiments. The results show that significant energy savings can be obtained.