To determine the extent to which Iberdrola should utilize hybrid bonds relative to S&P Global Ratings' cap, we must calculate the current percentage of hybrid bonds in its total adjusted capital and compare it to the 15% cap.

1. **Identify Hybrid Bonds:** In Iberdrola's balance sheet, hybrid bonds are classified under "Noncurrent Financial Liabilities" and "Current Financial Liabilities" as "Instrumentos De Capital Con Caracteristicas De Pasivo Financiero" (Capital instruments with financial liability characteristics). These are perpetual subordinated obligations, which align with S&P's definition of hybrid bonds receiving equity credit. 
   - Noncurrent hybrids: 576,000,000 EUR
   - Current hybrids: 87,000,000 EUR
   - **Total Hybrids:** 576M + 87M = **663,000,000 EUR**

2. **Identify Debt and Equity:** 
   - Total Equity (from "Equity"): **58,114,000,000 EUR**
   - Total Debt (Financial Liabilities):
     - Noncurrent Financial Liabilities: 44,216,000,000 EUR
     - Current Financial Liabilities: 25,079,000,000 EUR
     - **Total Debt:** 44,216M + 25,079M = **69,295,000,000 EUR**

3. **Calculate Total Adjusted Capital:**
   - Total Adjusted Capital = Debt + Equity = 69,295,000,000 + 58,114,000,000 = **127,409,000,000 EUR**

4. **Calculate Current Hybrid Usage Percentage:**
   - (Total Hybrids / Total Adjusted Capital) = 663,000,000 / 127,409,000,000 ≈ **0.52%**

5. **Determine the Room Under the Cap:**
   - S&P's maximum equity credit cap is 15% of total adjusted capital.
   - The company is currently utilizing less than 1% of its adjusted capital for hybrids, meaning there is substantial room (up to 14.5%) to issue additional hybrids before approaching the limit.
   - Because the current usage is far below the threshold, the company can fully utilize the available capacity under the cap. Therefore, they should utilize the cap to the maximum allowable extent (100% of the cap) if seeking to optimize their capital structure.

<answer>100%</answer>