To determine the optimal extent to which Acciona SA should utilize hybrid bonds relative to S&P Global Ratings' 15% cap, we need to compare the cost of hybrid bonds to the cost of equity and evaluate its effect on the Weighted Average Cost of Capital (WACC). 

1. **Calculate Total Adjusted Capital:**
   - **Total Equity** (2022 Year-End): €6,304,000,000
   - **Total Debt** = Non-current negotiable securities (€3,101M) + Non-current loans (€2,624M) + Non-current lease liabilities (€439M) + Current negotiable securities (€1,139M) + Current loans (€553M) + Current lease liabilities (€72M) = €7,928,000,000
   - **Total Adjusted Capital** (Debt + Equity) = €6,304M + €7,928M = €14,232,000,000
   - **S&P 15% Cap** = 15% * €14,232,000,000 = €2,134,800,000

2. **Estimate Cost of Hybrid Bonds:**
   - Yield on senior debt (ISHARES CORE EURO CORP BOND 2022 Average) = 1.085%
   - Sub-Senior Delta for Non-Financial IG = 2.295%
   - **Estimated Cost of Hybrid Debt** ≈ 1.085% + 2.295% = 3.38%

3. **Comparison with Cost of Equity:**
   - The cost of equity for an infrastructure, energy, and water conglomerate like Acciona typically runs well above 7-9% (depending on beta and equity risk premiums).
   - Because the cost of issuing hybrid bonds (~3.38%) is strictly and significantly cheaper than the firm’s Cost of Equity, replacing traditional equity with hybrid debt (which S&P credits as 50% equity up to the cap) brings down the firm's overall WACC without deteriorating the recognized credit metrics.

Therefore, to optimize its capital structure and lower its cost of capital while still protecting its credit rating, Acciona SA should maximize its usage of hybrid instruments up to the absolute limit allowed by the rating agency.

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