To determine the appropriate level of hybrid bond issuance for Naturgy Energy Group S.A., we must evaluate the company's financial position, leverage metrics, refinancing needs, and the potential benefits of equity credit from hybrids under the S&P Global Ratings methodology.

**1. Current Financial Position & Leverage Metrics:**
As of December 31, 2022, Naturgy reports the following key balance sheet figures:
- Total Equity: €9.979 billion
- Total Financial Debt: €14.799 billion (Calculated as Long-term borrowings €12.689B + Current borrowings €2.11B + Current lease liabilities €0.177B + Noncurrent lease liabilities €1.309B)
- Adjusted Debt (assuming S&P standard adjustments and hybrids as debt): ~€14.8 billion
- Total Adjusted Capital (Equity + Adjusted Debt): ~€24.8 billion
- Current Leverage (Debt / Capital): ~59.7%

For a regulated utility, S&P generally expects leverage (Debt/Capital) to be in the 40%-50% range for an 'A' category rating and 50%-60% for a 'BBB' category. Naturgy’s current leverage sits at the high end of the investment-grade spectrum. 

**2. S&P Equity Credit Cap:**
S&P limits the equity credit recognized from hybrid bonds to a maximum of 15% of total adjusted capital. 15% of €24.8 billion equals approximately €3.72 billion. Issuing hybrids up to this cap would increase adjusted capital to ~€28.5 billion while receiving 50% equity credit (standard for utility hybrids), thereby reducing the adjusted leverage ratio meaningfully (towards the mid-50s% range).

**3. Refinancing Needs:**
Naturgy has significant refinancing needs. In 2022, the company issued €783 million in new financial liabilities but repaid €1.625 billion. Current portion of noncurrent borrowings stands at €2.11 billion. In a rising interest rate environment (5Y swap curve moved from -0.264% in 2021 to 1.726% in 2022), refinancing this debt using standard senior unsecured bonds will increase the cost of debt. 

**4. Rationale for Hybrid Issuance:**
- **Leverage Optimization:** Given the 59.7% leverage, Naturgy has limited headroom at the current rating level. Issuing hybrids provides 50% equity credit under S&P methodology, which meaningfully improves adjusted leverage metrics and provides crucial rating flexibility.
- **Cost of Debt vs. Cost of Hybrids:** The cost of a hybrid bond is typically higher than senior debt. However, with the 5Y swap at 1.726% and sub-senior spreads for EUR IG non-financials at ~2.295%, the all-in cost of a hybrid would be roughly 4.0% - 4.5%. While this is slightly higher than the cost of senior debt, the equity credit benefit offsets this marginal increase by protecting the overall credit rating (and thus preventing the entire cost of capital from rising due to a downgrade). 
- **Capital Structure:** The company does not appear to have any outstanding hybrid bonds currently treated as equity. Introducing a moderate layer of hybrids is a standard and efficient tool for integrated utilities to manage leverage without diluting shareholders.

**Conclusion:**
Naturgy does not face "transformational" capex or material downgrade risk that would push it to the absolute 15% cap immediately (which would imply a €3.7 billion issuance). However, its leverage is moderately high, and it has moderate refinancing needs in a higher rate environment. Issuing hybrids at the 7.5% threshold of total adjusted capital (~€1.86 billion, well within the €3 billion annual cap) offers a strong, meaningful improvement to adjusted leverage, provides useful rating headroom, and optimizes the capital structure without the cost of hybrids disproportionately impacting the cost of debt. 

<answer>7.5%</answer>