To determine the extent to which NATURGY ENERGY GROUP SA should utilize hybrid bonds relative to S&P Global Ratings' 15% cap, we need to compare the cost of issuing hybrid bonds to the combined cost of the debt and equity they replace. S&P typically grants 50% equity credit to hybrid bonds, meaning 2 units of hybrid debt can replace 1 unit of senior debt and 1 unit of equity while maintaining the same adjusted credit metrics.

The company should maximize its hybrid utilization (100% of the cap) if the cost of 2 units of hybrid debt is less than the cost of 1 unit of senior debt plus 1 unit of equity. Mathematically, this condition is:
2 * k_h < k_d + k_e 
Which simplifies to: 
k_e > k_d + 2 * Δ (where Δ is the yield premium of hybrids over senior debt)

1. **Cost of Debt (k_d) and Hybrid Premium (Δ):**
Using the 2022 market data, the average yield for Investment Grade (IG) Corporate Bonds (k_d) is 1.085%. 
The spread or premium for Subordinated/Hybrid bonds (Δ) is 2.295%.
Minimum required k_e = 1.085% + 2 * (2.295%) = 5.675%.

2. **Cost of Equity (k_e):**
We can estimate the implied cost of equity using the company's Return on Equity (ROE). 
Average Equity for 2022 = (7,574,000,000 EUR + 5,889,000,000 EUR) / 2 = 6,731,500,000 EUR.
Net Income Attributable to Owners = 1,649,000,000 EUR.
ROE = 1,649,000,000 / 6,731,500,000 ≈ 24.50%.

Since the company's cost of equity (~24.50%) is vastly greater than the minimum threshold required for hybrid issuance to be cost-effective (5.675%), replacing common equity and senior debt with hybrid bonds is heavily accretive to the firm's overall cost of capital. 

Therefore, to minimize its Weighted Average Cost of Capital (WACC) while preserving its credit rating, NATURGY ENERGY GROUP SA should utilize hybrid bonds up to the absolute maximum allowed by S&P's equity credit limits.

<answer>100%</answer>