To determine the optimal extent to which the company's capital structure should rely on hybrid bonds, we can look at the theoretical efficiency of hybrid securities compared to standard instruments. 

While Enel currently has a portion of its capital structure in perpetual hybrid bonds (valued at approximately €5.57 billion at the end of 2022, which represents a relatively small fraction of its total capitalization), theoretical corporate finance models generally discourage a heavy reliance on them. 

Hybrid bonds (such as convertible debt, perpetual bonds, or subordinated debt with equity-like features) are structured to contain characteristics of both pure debt and pure equity. However, they almost always carry a **complexity premium** and an **illiquidity premium** compared to plain vanilla bonds or standard common equity. Because any hybrid instrument's payoff profile can essentially be replicated using a tailored mix of straight, senior debt and common equity—usually at a lower combined issuance and yield cost—it is mathematically inefficient to use them as a cornerstone of a firm's capital structure.

Furthermore, rating agencies typically cap the "equity credit" they assign to hybrid bonds (often up to a maximum of 15-25% of total capital before treating the rest purely as debt), meaning their structural benefits run out quickly. Straight debt offers a clearer, generally cheaper tax shield, while common equity provides pure financial flexibility without fixed-coupon obligations. 

Therefore, from a purely optimal capital structure perspective, a company should minimize its reliance on hybrid bonds in favor of a balanced mix of traditional debt and equity.

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