To determine the appropriate extent to which Enel SpA's capital structure should rely on hybrid bonds, we need to analyze the company's financial data and the economic environment provided:

1. **Current Capital Structure and Hybrid Bond Usage**: Looking at Enel's 2022 balance sheet, the total equity is 42.08 billion EUR and total liabilities are 177.54 billion EUR (total capitalization of ~219.6 billion EUR). The specific line item "Equity Instruments Perpetual Hybrid Bonds Member" shows a balance of 5.567 billion EUR. This represents roughly 2.5% of total capitalization and around 13% of total equity. The company has already utilized hybrid bonds to a modest degree, recognizing their utility as a hybrid instrument that counts toward equity credit rating buffers while being economically similar to subordinated debt.

2. **Interest Rate Environment**: The provided market data shows a dramatic shift in the interest rate landscape from 2021 to 2022. The 5Y swap curve moved from an average of -0.264% in 2021 to 1.726% in 2022, and the 10Y swap curve moved from 0.053% to 1.927%. Similarly, corporate bond yields (iShares Core Euro Corp Bond) increased significantly. In this sharply rising rate environment, locking in long-term perpetual hybrid bonds becomes substantially more expensive than it was in previous years, creating a natural deterrent to over-relying on this instrument.

3. **Credit Rating and Optimal Structure**: Utility companies like Enel typically target a solid investment-grade rating (e.g., BBB+ to A-), which necessitates a balanced and resilient capital structure. While hybrid bonds are beneficial for equity credit and substituting long-term debt without diluting shareholders, agencies (like S&P and Fitch) cap the equity credit they afford (often treating them as 50% equity at the 5-7 year point). Over-relying on hybrids (e.g., 50% or more of the capital structure) would fail to provide the desired equity credit and would excessively burden the company with high-cost, subordinated obligations in a rising rate market.

4. **Strategic Assessment**: Given the current interest rates, the rising cost of capital, and the regulatory caps on equity recognition, the firm should rely on hybrid bonds for a supplementary portion of its capital rather than a primary one. The existing 13% of equity / ~2.5% of total capital is typical for European utilities seeking a slight leverage adjustment. A 25% reliance provides a meaningful buffer—enhancing the balance sheet and rating agency equity cushion—without over-exposing the firm to punitive subordinated rates or stepping beyond the limits of agency equity recognition. Anything higher (50%, 75%, 100%) would be inconsistent with utility finance norms and damaging in the current rate environment.

<answer>25%</answer>