I need to analyze whether Naturgy Energy Group should utilize hybrid bonds up to S&P Global Ratings' 15% cap of total adjusted capital.

First, let me understand what hybrid bonds are and why companies use them:
- Hybrid bonds have both debt and equity characteristics
- They receive partial equity credit from rating agencies (typically 50-100% equity treatment depending on structure)
- S&P limits equity credit on hybrids to maximum 15% of total adjusted capital (debt + equity)

Let me examine Naturgy's capital structure from the financial data:

**As of 2023-01-01 (end of 2022):**
- Equity: 9,979,000,000 EUR
- Noncurrent Financial Liabilities: 13,999,000,000 EUR
- Current Financial Liabilities: 2,302,000,000 EUR
- Total Debt: ~16,301,000,000 EUR

**As of 2022-01-01 (end of 2021):**
- Equity: 8,873,000,000 EUR
- Noncurrent Financial Liabilities: 15,114,000,000 EUR
- Current Financial Liabilities: 1,698,000,000 EUR
- Total Debt: ~16,812,000,000 EUR

Let me calculate key metrics to assess Naturgy's financial position and need for hybrid capital:

**Leverage Ratios (2022):**
- Total Adjusted Capital = Debt + Equity = 16,301 + 9,979 = 26,280 million EUR
- Debt/Equity = 16,301 / 9,979 = 1.63x
- Debt / Total Capital = 16,301 / 26,280 = 62%

**Key Considerations for Hybrid Bonds:**

1. **Current Leverage Position**: With debt at ~62% of total capital, Naturgy has significant leverage. This suggests potential value in optimizing capital structure.

2. **Credit Rating Considerations**: 
   - Hybrid bonds provide equity credit, improving leverage metrics used by rating agencies
   - S&P's 15% cap means maximum equity credit from hybrids = 15% × (Debt + Equity)

3. **Interest Rate Environment (2022)**:
   - Swap rates turned significantly positive in 2022 (5Y: 1.726%, 7Y: 1.806%, 10Y: 1.927%)
   - Corporate bond spreads were elevated (iBoxx non-financial IG at 2.295%)
   - This makes traditional debt more expensive, increasing relative attractiveness of hybrids

4. **Company Characteristics**:
   - Utilities/energy companies are typical hybrid issuers due to stable cash flows
   - Naturgy has significant infrastructure assets (PPE: 17.4 billion EUR)
   - Regulated utility operations provide stable cash flows suitable for hybrid coupon payments

5. **Equity Credit Optimization**:
   - Maximum 15% equity credit from hybrids
   - If Naturgy wants to maximize rating benefit, it should use the full 15% capacity
   - However, hybrids are typically more expensive than senior debt

6. **Market Context in 2022**:
   - Rising interest rates increase cost of all financing
   - Hybrids may offer relative cost advantage when equity markets are volatile
   - Naturgy's equity increased from 8.9bn to 10.0bn, suggesting retained earnings strength

**Analysis of Utilization Levels:**

- **0%**: Would ignore valuable equity credit opportunity, especially with elevated leverage
- **25%**: Conservative, but underutilizes available equity credit capacity
- **50%**: Moderate usage, captures meaningful equity credit while maintaining flexibility
- **75%**: Strong utilization, significant equity credit benefit
- **100%**: Maximum utilization of the 15% cap, full optimization of rating agency equity credit

Given:
- Rising rate environment making traditional debt more expensive
- Utility sector suitability for hybrid instruments
- Significant existing leverage (62% debt/capital)
- Stable cash flow generation (EBITDA 4.95 billion EUR)
- Need to maintain investment grade rating

The optimal strategy is to **fully utilize** the 15% cap. This provides:
- Maximum equity credit for rating purposes
- Diversification of funding sources
- Potential cost savings vs. pure equity
- Financial flexibility

Utilities typically benefit most from full hybrid utilization due to their stable, regulated cash flows and need for substantial capital investment. Naturgy's infrastructure-heavy balance sheet and regulated activities make it an ideal candidate for maximum hybrid utilization.

<answer>100%</answer>