To determine the appropriate extent to which NATURGY ENERGY GROUP SA's capital structure should rely on hybrid bonds, we must analyze the company's financial profile, the nature of its industry, and the strategic role hybrid bonds play for utility companies.

1. **Industry Profile and Credit Stability**: Naturgy is a major regulated utility company (gas and electricity). Such companies are characterized by highly predictable and stable cash flows, heavy capital expenditure requirements, and a strategic need to maintain strong investment-grade credit ratings. 
2. **Current Capital Structure**: Looking at the 2022 year-end balance sheet, the company has Total Equity of €9.979 billion and Total Liabilities of €30.411 billion. Noncurrent and current financial liabilities (debt) amount to roughly €16.3 billion. The existing structure already employs a significant amount of leverage, which is typical for the sector.
3. **Role of Hybrid Bonds**: Hybrid bonds (such as perpetual subordinated bonds) possess equity-like features because they are deeply subordinated and can be written down or have coupons skipped without triggering a default. Rating agencies (like Fitch, Moody's, and S&P) treat a significant portion (usually 50%) of hybrid bonds as "equity credit," which allows utilities to raise capital without diluting common shareholders while still improving their debt-to-equity metrics and protecting their investment-grade ratings.
4. **Optimal Reliance (25% vs. 50% vs. Higher)**: While hybrids are highly beneficial for utilities, they are expensive due to their subordinated risk profile (higher coupon demands). Thus, they are used as a supplement rather than a core funding source. Relying on hybrids for 50% or more of the capital structure would be excessively costly, highly subordinated, and unconventional even for the most aggressive regulated utilities. A 25% weighting in the total capital structure is the industry "sweet spot" for regulated utilities—it allows the company to reap the rating agency equity credit benefits, maintain a robust traditional debt stack (senior secured/senior unsecured), and optimize the weighted average cost of capital (WACC) without overpaying for capital. A 0% reliance would leave rating agency enhancement on the table, which is suboptimal for a leveraged utility.

Therefore, a moderate but meaningful reliance of 25% is the most appropriate target for a regulated utility like Naturgy to balance senior debt, common equity, and hybrid instruments effectively.

<answer>25%</answer>