The knowledge of the electric field created by microwave power injected inside the vacuum vessel is fundamental to understand second harmonic ECRH plasma breakdown in magnetic confinement devices. The simple photon model described in this letter allows to determine the electric field assuming that a homogeneous and stationary energy density state is reached during the first nanoseconds after the wave injection. According to second harmonic breakdown theory, the measurement of the maximum prefill pressure for which breakdown is achieved allows an indirect determination of the required electric field strength. Experiments performed in HSX show that the field value obtained in this way is in very good agreement with the value calculated with the model.