Hybrid bonds are typically used by companies that want to strengthen their credit metrics without issuing additional common equity, because they combine features of both debt and equity (often treated as partial equity by rating agencies).  

To assess the extent to which Naturgy should rely on hybrid bonds, key factors are:

1. **Credit quality and leverage** – From the balance sheet at end-2022:
   - Total equity = €9,979M  
   - Noncurrent financial liabilities = €13,999M, of which long-term borrowings = €12,689M  
   - Total debt-like items are significant, but equity is also substantial. The leverage ratio (Debt/Equity) is elevated but manageable for a utility.

2. **Earnings stability and cash flow** – EBITDA in 2022 = €4,954M, interest cost = €837M. Interest coverage is strong (≈5.9x EBITDA/Finance costs). Operating cash flow = €4,242M. This suggests a relatively stable and cash-generative business, typical for an energy utility.

3. **Market conditions** – Swap rates moved sharply up in 2022 (e.g., 10Y from 0.053% in 2021 to 1.927% in 2022). Corporate bond yields also rose (iBoxx EUR Non-Fin IG sub-senior delta average 2.295% in 2022). Rising rates increase the cost of hybrid issuance, making it less attractive compared to fixed-rate senior debt or retained earnings.

4. **Equity vs. hybrid trade-off** – Hybrids are more expensive than senior debt; they are mainly used when equity issuance is too dilutive or impossible, and when the company needs to defend its credit rating. Naturgy’s equity is positive and sizable, and they are already paying dividends (€1.2/share in 2022). Extreme reliance (75%–100%) is unusual and risky. Zero reliance is also suboptimal, as a small hybrid portion can optimize the cost of capital and rating headroom for a capital-intensive utility.

Given Naturgy’s stable cash flows, reasonable leverage, and rising rate environment, a moderate use (around 25%) would make sense to balance cost and credit metrics, but not a high proportion.

<answer>25%</answer>