I need to analyze ENEL's capital structure and determine the appropriate reliance on hybrid bonds. Let me examine the relevant data.

## Key Data Points:

### Hybrid Bonds in Equity Structure:
- 2021: Equity Instruments Perpetual Hybrid Bonds = 2,386,000,000 EUR
- 2022: Equity Instruments Perpetual Hybrid Bonds = 5,567,000,000 EUR
- 2023: Equity Instruments Perpetual Hybrid Bonds = 5,567,000,000 EUR

### Total Equity Structure:
- 2021: Total Equity = 42,357,000,000 EUR; Equity Attributable to Owners = 28,325,000,000 EUR
- 2022: Total Equity = 42,342,000,000 EUR; Equity Attributable to Owners = 29,653,000,000 EUR
- 2023: Total Equity = 42,082,000,000 EUR; Equity Attributable to Owners = 28,657,000,000 EUR

### Hybrid Bonds as % of Equity Attributable to Owners:
- 2021: 2,386 / 28,325 = 8.4%
- 2022: 5,567 / 29,653 = 18.8%
- 2023: 5,567 / 28,657 = 19.4%

### Total Capital Structure (Debt + Equity):
- 2023: Total Assets = 219,618,000,000 EUR; Equity = 42,082,000,000 EUR; Liabilities = 177,536,000,000 EUR

### Long-term Borrowings:
- 2023: 68,191,000,000 EUR (including 774M related party)
- 2022: 54,500,000,000 EUR

## Analysis:

1. **Current Hybrid Bond Usage**: ENEL currently has hybrid bonds at roughly 19-20% of equity attributable to owners, or about 13% of total equity. This is a moderate level.

2. **Benefits of Hybrid Bonds for ENEL**:
   - Equity credit from rating agencies (typically 50-100% equity treatment)
   - Lower cost than pure equity
   - No maturity date (perpetual)
   - Interest can be deferred (typical feature)

3. **Market Conditions (2022)**:
   - Interest rates rose significantly (swap curves turned positive after negative/flat period)
   - 5Y swap: from -0.264% (2021) to 1.726% (2022)
   - 10Y swap: from 0.053% (2021) to 1.927% (2022)
   - Corporate bond spreads: iShares Core Euro Corp Bond at 1.085% (2022) vs 0.733% (2021)

4. **Rising Rate Environment Impact**:
   - Higher interest rates make hybrid bonds more expensive to issue
   - However, they still offer cost advantage over pure equity
   - The "bull" scenario shows rates could go higher (1.426% 5Y, 1.627% 10Y)

5. **ENEL's Financial Position**:
   - Significant debt load (177.5B liabilities vs 42B equity)
   - Debt-to-equity ratio is already high (~4.2x)
   - Need to maintain investment grade rating
   - Discontinued operations loss in 2022 (-2.3B EUR)

6. **Optimal Hybrid Bond Level Considerations**:
   - Too low (0%): Misses cost savings and equity credit benefits
   - Too high (75-100%): Risk of reclassification by rating agencies, coupon deferral risk, investor concerns
   - Rating agencies typically give 50% equity credit up to certain thresholds (often 10-15% of total capitalization)

7. **Industry Practice**:
   - Utilities typically use hybrids at 5-15% of total capitalization
   - ENEL is already at the upper end with ~19% of equity attributable
   - Given rising rates and already high leverage, moderate increase is warranted

## Assessment:

ENEL should **increase** hybrid bond usage from current levels but not excessively. Given:
- High existing leverage needs equity credit
- Rising rate environment makes pure debt expensive
- Need to maintain financial flexibility
- Rating agency considerations suggest 50% equity treatment is optimal when hybrids are 10-20% of total cap

A target of **25%** of equity attributable to owners in hybrid bonds would be prudent - this provides meaningful equity credit and cost savings without risking reclassification or excessive financial complexity. This represents an increase from current ~19% but not an extreme level.

<answer>25%</answer>