<p>Two known methods of modeling a free-piston Stirling engine are considered: in the first, the influence of the electric generator is taken into account as a simple friction force in mechanics, while in the second, the generator and its electric load are included in the model in an explicit form. Contrary to the current concepts, the results obtained differ even at steady-state engine operation with identical frequencies. To get a reliable design of the "Stirling engine–electric generator–load (consumer)" system, it is necessary to use an expression for the "friction" force that depends on the electric current, i.e., modeling the current source and load is prerequisite, thereby making it possible to take into account their inverse influence on engine operation. This is of fundamental importance, since in practice the power of the consumer is often comparable to the power of the Stirling engine.</p>