To determine the optimal reliance on hybrid bonds for ACEA S.P.A.'s capital structure, we evaluate the trade-offs between the costs of various sources of financing and their respective tax advantages. 

Hybrid bonds incorporate characteristics of both debt and equity. While they provide a tax shield similar to standard debt (since interest payments are typically tax-deductible), they are subordinated to senior debt. This subordination subjects hybrid bondholders to higher risk in the event of default or bankruptcy, which is reflected in a higher yield compared to senior unsecured debt. The provided market data clearly shows a positive "Sub-Sen Delta" for standard non-financial investment-grade entities (e.g., reaching 2.295% in 2022 on average), meaning the cost of issuing hybrid/subordinated debt is significantly higher than that of senior debt (iShares Core Euro Corp Bond average was 1.085% in 2022). 

In standard capital structure optimization (such as minimizing the Weighted Average Cost of Capital, or WACC), hybrid bonds are generally strictly dominated by a combination of cheaper senior debt and common equity. Unless the company is up against strict debt-capacity ceilings, specific credit rating agency constraints, or regulatory requirements—which are primarily binding for financial institutions rather than non-financial utility companies like ACEA—there is no cost advantage to utilizing hybrid bonds. Introducing them would unnecessarily increase the firm's cost of capital without an offsetting benefit that couldn't be achieved more efficiently through standard senior debt. 

Therefore, in a purely value-maximizing framework, a non-financial company like ACEA S.P.A. should not allocate any portion of its target capital structure to hybrid bonds.

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