To determine the extent to which Naturgy Energy Group should utilize hybrid bonds relative to the S&P Global Ratings' cap, we must first calculate the maximum amount of hybrid bonds allowed under the cap. S&P limits the equity credit on hybrids to a maximum of 15% of total adjusted capital, which is defined as Debt + Equity.

1. **Calculate Total Equity:**
From the balance sheet as of 2023-01-01:
Total Equity = 9,979,000,000 EUR

2. **Calculate Total Debt:**
We sum the noncurrent and current financial liabilities (excluding lease liabilities, as S&P's 15% cap applies to financial debt, and hybrids are a substitute for traditional financial debt):
- Longterm Borrowings = 12,689,000,000 EUR
- Other Noncurrent Financial Liabilities = 1,000,000 EUR
- Current Borrowings And Current Portion Of Noncurrent Borrowings = 2,110,000,000 EUR
- Other Current Financial Liabilities = 15,000,000 EUR
Total Financial Debt = 12,689,000,000 + 1,000,000 + 2,110,000,000 + 15,000,000 = 14,815,000,000 EUR

3. **Calculate Total Adjusted Capital:**
Total Adjusted Capital = Total Financial Debt + Total Equity 
Total Adjusted Capital = 14,815,000,000 EUR + 9,979,000,000 EUR = 24,794,000,000 EUR

4. **Calculate the S&P Hybrid Cap (15% of Total Adjusted Capital):**
S&P Hybrid Cap = 15% × 24,794,000,000 EUR = 3,719,100,000 EUR

5. **Evaluate the Extent of Utilization:**
The question asks to what extent the company should utilize hybrid bonds relative to this cap based on the provided facts. Utilities like Naturgy typically issue hybrid bonds for credit rating optimization (to gain equity credit while maintaining the tax shield of debt). As long as the issuance is well within the S&P cap to ensure full equity treatment and does not breach restrictive covenants, maximizing the use of this cheap, equity-credit-eligible instrument is optimal for minimizing the Weighted Average Cost of Capital (WACC). Since the theoretical cap allows for a substantial buffer, and there are no indicators of constraints limiting this strategy, the company should utilize the allowance to its fullest extent—100% of the cap.

<answer>100%</answer>