To determine the extent to which Enel should utilize hybrid bonds relative to the S&P Global Ratings' cap, we first need to calculate the current proportion of hybrid bonds within the company's total adjusted capital and compare it to the 15% limit.

1. **Identify Total Debt:**
   From the balance sheet (as of 2023-01-01):
   - Long-term borrowings = €68,191,000,000
   - Short-term borrowings = €18,392,000,000
   - Current portion of long-term borrowings = €2,835,000,000
   - Total Debt = €68,191M + €18,392M + €2,835M = €89,418,000,000

2. **Identify Total Equity:**
   From the balance sheet (as of 2023-01-01):
   - Total Equity = €42,082,000,000

3. **Identify Hybrid Bonds (Equity Credit):**
   From the statement of changes in equity (as of 2023-01-01):
   - Equity Instruments - Perpetual Hybrid Bonds = €5,567,000,000

4. **Calculate Total Adjusted Capital:**
   - Total Adjusted Capital = Total Debt + Total Equity
   - Total Adjusted Capital = €89,418M + €42,082M = €131,500,000,000

5. **Calculate the Current Hybrid Bond Utilization:**
   - Proportion = Hybrid Bonds / Total Adjusted Capital
   - Proportion = €5,567M / €131,500M ≈ 4.23%

6. **Compare to S&P Cap:**
   - S&P Cap = 15%
   - Current Utilization (4.23%) is significantly below the 15% threshold (it represents about 28% of the maximum allowed capacity). 

Because the company's current usage of hybrid bonds is far below the 15% cap, they have substantial headroom to issue additional hybrid bonds if they wish to optimize their capital structure, lower their cost of capital, or support their credit rating by utilizing the full equity credit capacity allowed by S&P. The extent to which they *should* utilize this instrument relative to the cap leans towards a significant portion given their active use and the strategic benefits in the utilities sector, but since they currently only utilize about a quarter of the cap, the extent to which they *should* utilize it (to reach their optimal target relative to the max) points toward 25% of the cap being their current effective benchmark relative to the limit.

<answer>25%</answer>