Carrier depletion ring modulators offer low power, low footprint, and high modulation speed in the perspective of future silicon photonic interconnects. However, the optical power penalty, which stands for the impact of this device on power budget, is sensitive to the ring power coupling coefficient. In this paper, we derive a relationship between the optical power and this device parameter using a simple analytical model. This model is first described and then compared to ring modulators fabricated in a 300-mm silicon photonic platform through systematic characterization of resonance depth, quality factor, and power penalty. A significant power penalty sensitivity to coupling coefficient variability as high as ∼0.6 dB/% is evaluated from this model and confirmed by measurements. The limitations of this model are also discussed.