To determine the optimal extent to which IBERDROLA SA should utilize hybrid bonds relative to S&P Global Ratings' 15% cap on total adjusted capital, we compare the cost of hybrid bonds to the cost of equity.

**1. Cost of Debt and Hybrids:**
Based on the market data provided for 2022:
- The yield on senior unsecured debt (proxy for the cost of standard debt, $K_d$) is approximately the ISHARES CORE EURO CORP BOND yield: **1.085%**.
- The hybrid bond cost ($K_h$) incorporates the subordinated-senior delta. Adding the SUB-SEN DELTA FOR IBOXX EUR NON-FINANCIAL IG (2.295%) to the senior yield gives an estimated hybrid yield of:
  $K_h = 1.085% + 2.295% = 3.380%$

**2. Cost of Equity:**
The risk-free rate is approximated by the 10-year swap curve average for 2022, which is **1.927%**. Assuming a standard equity risk premium of ~5-6% and a utility beta of around 0.8:
- $K_e \approx 1.927% + 0.8 \times 5.5% \approx 6.33%$

**3. Decision Rule:**
Hybrid bonds receive 50% equity credit and 50% debt credit from S&P. Because the after-tax cost of hybrid bonds (even if their coupons are treated as non-tax-deductible dividends) is substantially lower than the cost of equity ($3.38\% < 6.33\%$), issuing hybrids is a cheaper way to support the company's credit rating than issuing common equity. 

In order to minimize its Weighted Average Cost of Capital (WACC) while maintaining target rating metrics, a company should fully maximize its use of cheaper equity-credited instruments before resorting to standard common equity. Therefore, Iberdrola should fill the entire capacity allowed by the rating agency.

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