To determine the extent to which HERA S.P.A.'s capital structure should rely on hybrid bonds, we evaluate the company's financial profile, sector characteristics, and standard credit rating agency methodologies. 

1. **Industry and Capital Intensity**: HERA S.P.A. operates as a multi-utility company (Gruppo Hera) primarily in Italy. The utility sector is inherently capital-intensive, requiring continuous investment in infrastructure. This is evident from their high non-current assets (7.9 billion EUR in 2022) and high capital expenditures.

2. **Leverage and Capital Structure**: HERA’s Total Liabilities at the end of 2022 stood at roughly 13.47 billion EUR, compared to a Total Equity of 3.64 billion EUR. This results in a relatively high Debt-to-Equity ratio (approx. 3.7x). High leverage often puts pressure on corporate credit ratings.

3. **Interest Coverage and Profitability**: HERA's operating profit (EBIT) was 533.8 million EUR, and its finance costs were 217.2 million EUR, yielding an Interest Coverage Ratio of approximately 2.45x. While this coverage is adequate, the company's free cash flow generation has been constrained by heavily negative changes in working capital in 2022 (cash from operations dropped to 35.7 million EUR).

4. **Hybrid Bonds Rationale**: Hybrid bonds are deeply subordinated debt instruments that rating agencies (such as S&P, Moody's, and Fitch) treat partially as equity (typically giving 50% equity credit) when calculating credit metrics. Utilities commonly issue hybrid bonds to fund capital expenditures and acquisitions without diluting shareholders or risking a credit rating downgrade from issuing pure senior debt. 

However, rating agencies generally cap the amount of hybrid capital that can receive equity treatment at **25%** of a company's total capitalization. Relying on hybrid bonds beyond this 25% limit would mean the excess is treated as 100% debt, stripping away the rating benefits while leaving the company to pay the higher hybrid interest premiums.

Given HERA's high leverage, its sector (utilities), and standard rating agency limits, 25% is the optimal maximum reliance on hybrid bonds to bolster equity credit while managing the cost of capital.

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