Let’s reason through this step-by-step.

**1. Understanding hybrid bonds in capital structure**  
Hybrid bonds have characteristics of both debt and equity. They can improve credit ratios and provide tax benefits (interest is tax-deductible), but they also come with higher coupon costs than senior debt and can be called or converted.

**2. ENEL’s current use of hybrids**  
From the equity breakdown:  
- 2022-01-01: Equity Instruments Perpetual Hybrid Bonds = €5,567M  
- 2023-01-01: Equity Instruments Perpetual Hybrid Bonds = €5,567M  
- Total equity attributable to owners of parent: ~€28.7B  
So hybrids are about 19.4% of parent equity and about 13.2% of total equity.  
This suggests ENEL is already using hybrids but not as the dominant source.

**3. Debt levels and leverage**  
- Total liabilities are high (€177.5B at end of 2022).  
- Long-term borrowings increased from €54.5B to €68.2B year-over-year.  
- Operating profit improved, but profit after tax fell significantly due to discontinued operations and impairments.  
This indicates some caution is needed in adding more fixed-cost capital.

**4. Market environment in 2022**  
- Swap rates rose sharply in 2022 (5Y from negative to +1.726%).  
- Corporate bond yields also rose (iBoxx sub-senior IG from 1.298% to 2.295%).  
- This means issuing hybrid bonds became more expensive.  
- A heavy reliance on hybrids (75–100%) would lock in high coupon costs for a long period.

**5. Strategic considerations**  
- 0% would ignore the benefits of hybrid capital for credit rating optimization.  
- 100% would be very aggressive and likely too costly and risky.  
- 25% to 50% seems reasonable—provides flexibility without overburdening interest costs.  
- Given ENEL already sits near ~19% of parent equity in hybrids, moving toward 25% is a modest increase, whereas 50% would mean roughly doubling hybrid reliance, which seems aggressive given the rising rate environment.

**6. Conclusion**  
A 25% reliance strikes a balance between benefiting from hybrid features and managing the cost of capital in a rising rate environment.

<answer>25%</answer>