I need to analyze ENGIE's capital structure and determine the appropriate reliance on hybrid bonds.

## Key Analysis Points

### 1. Current Capital Structure Analysis

From the balance sheet data:
- **Equity**: €39.285 billion (2023), €41.98 billion (2022)
- **Long-term borrowings**: €28.083 billion (2023), €30.458 billion (2022)
- **Current borrowings**: €12.508 billion (2023), €10.59 billion (2022)
- **Total debt**: ~€40.6 billion (2023), ~€41 billion (2022)

The company already has significant debt and uses deeply subordinated perpetual notes (a form of hybrid capital):
- Deeply Subordinated Perpetual Notes: €3.393 billion (2023), €3.767 billion (2022), €3.913 billion (2021)

This represents roughly 8-9% of total equity, showing ENGIE already uses some hybrid instruments.

### 2. Financial Performance and Stability

**Profitability concerns:**
- 2022: Profit Loss of only €390 million (vs €3.758 billion in 2021)
- Continuing operations actually lost €1.793 billion in 2022
- The profit was saved by discontinued operations (€2.183 billion)
- Current Operating Income dropped from €6.116 billion to €4.309 billion

**Cash flow:**
- Operating cash flow: €8.586 billion (2022), €7.313 billion (2021) - relatively healthy
- Free cash flow after capex: ~€2.2 billion

**Leverage indicators:**
- Assets: €235.49 billion; Equity: €39.285 billion
- Equity/Assets ratio: ~16.7% (quite low, indicating high leverage)

### 3. Market Environment for Hybrid Bonds

From the swap curve data:
- **2022 saw dramatic rate increases**: 5Y swap went from -0.264% to 1.726%, 10Y from 0.053% to 1.927%
- Corporate bond spreads (iShares Core Euro Corp Bond): relatively stable ~1%
- Sub-senior delta for non-financial IG: ~2.3% in 2022

This rising rate environment makes fixed-income instruments more expensive to issue.

### 4. Hybrid Bond Suitability Assessment

**Arguments for hybrid bonds:**
- Already used by ENGIE (perpetual notes exist)
- Utilities typically can support hybrid capital due to stable cash flows
- Tax efficiency of interest payments vs dividends
- Equity credit from rating agencies (typically 50-100% equity treatment)

**Arguments against heavy reliance:**
- ENGIE's profitability is volatile (2022 showed major earnings decline)
- Already highly leveraged (equity/assets only ~17%)
- Hybrid bonds are complex and expensive in rising rate environments
- The company's credit quality may be pressured (negative continuing operations profit)
- Discontinued operations boosted 2022 results - not sustainable

**Utility sector considerations:**
- Utilities traditionally use moderate leverage with stable regulated cash flows
- However, ENGIE has significant merchant/unregulated exposure (energy trading, renewables development)
- The 2022 results show vulnerability to commodity price volatility

### 5. Optimal Hybrid Capital Level

Given:
- ENGIE already uses ~8-9% hybrid capital (perpetual notes/equity)
- The company is not a pure regulated utility (more integrated/merchant exposure)
- Profitability is volatile and 2022 showed stress
- Overall leverage is already high
- Rising rate environment increases cost

For a company with these characteristics:
- **0%**: Too conservative, misses benefits of tax-efficient capital
- **100%**: Extreme, would destroy credit quality and rating
- **75%**: Far too high for a volatile utility
- **50%**: Very high, would imply majority of "equity" is actually debt-like
- **25%**: Moderate level - provides some diversification and tax benefit without excessive risk

A 25% hybrid share of total capital (or roughly 50% equity treatment from rating agencies) is typical for utilities with some merchant exposure. This provides:
- Meaningful tax shield benefit
- Some equity credit for ratings
- Not so much that financial flexibility is severely impaired if markets close

Given ENGIE's volatile earnings, high existing leverage, and mixed regulated/merchant business model, a moderate hybrid reliance is appropriate - higher than 0% but well below levels suitable for pure regulated utilities.

<answer>25%</answer>