To determine the extent to which ENGIE's capital structure should rely on hybrid bonds, we must evaluate the company's current financial position, its industry norms, and the strategic role of hybrid capital.

1. **Current Capital Structure & Industry Context**: ENGIE is a major utility company operating in energy transition and infrastructure. Utilities typically carry significant debt due to the capital-intensive nature of their business. At the end of 2022, ENGIE reported Total Equity of €39.285 billion (including €5.032 billion in noncontrolling interests and €3.393 billion in deeply subordinated perpetual notes, which act as hybrid equity) and Noncurrent Liabilities (primarily long-term borrowings and derivatives) of €102.427 billion, alongside Current Liabilities of €93.778 billion. 

2. **Existing Hybrid Capital**: The balance sheet already shows "Deeply Subordinated Perpetual Notes Member" valued at €3.393 billion, which are essentially hybrid bonds. This represents roughly 8-9% of their total equity and a smaller fraction of their total capitalization. The existence of these notes shows management's historical willingness to use hybrid instruments, which receive partial equity credit from rating agencies (like S&P and Moody's) and help maintain investment-grade metrics while optimizing the cost of capital.

3. **Credit Metrics and Debt Refinancing**: In 2022, rising interest rates significantly impacted the company's finance costs (up from €2.061B to €3.7B) and caused massive mark-to-market losses on derivatives, driving net financial loss to -€3.003B and severely compressing net income. To manage debt maturities and high refinancing needs (€10.972B in borrowings repaid in 2022), utilities often use hybrid bonds to artificially bolster their equity base without diluting shareholders, thereby satisfying rating agency down-stops.

4. **Appropriate Reliance**: While hybrid bonds are useful for optimizing the weighted average cost of capital (WACC) and keeping leverage ratios in check, they are deeply subordinated, carry higher coupon rates, and are not a substitute for true equity or core debt. A reliance of 25% on hybrid bonds within the capital structure (considering a mix of equity + hybrids + traditional debt) represents a balanced, aggressive but sustainable optimization. It provides a meaningful equity cushion for rating agencies while avoiding the excessive fixed-charge burden and refinancing risk that would come with 50%+ reliance. For a BBB-rated utility, a 25% weighting of hybrid securities relative to its equity base, or as a strategic target tranche in the capital structure, aligns perfectly with standard corporate finance optimization without jeopardizing financial stability.

<answer>25%</answer>