Electric circuits with transient elements can be good examples of systems where non-steady irreversible processes occur; so in the same way as a steady-state energy converter, we use the formal construction of the first-order irreversible thermodynamic to describe the energetics of these circuits. In this case, we propose an isothermal model of two meshes with transient and passive elements, besides containing two voltage sources (which can be functions of time); this is a non-steady energy converter model. Through the Kirchhoff equations, we can write the circuit phenomenological equations. Then, we apply an integral transformation to linearize the dynamic equations and rewrite them in algebraic form, but in the frequency space. However, the same symmetry for steady states appears (cross effects). Thus, we can study the energetic performance of this converter model by means of two parameters: the "force ratio" and the "coupling degree". Furthermore, it is possible to obtain characteristic functions (dissipation function, power output, efficiency, etc.). They allow us to establish a simple optimal operation regime of this energy converter. As an example, we obtain the converter behavior for the maximum efficient power regime.