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

**1. Assess HERA's capital structure and leverage:**

From the balance sheet at 2023-01-01:
- Total equity = €3,644.7 million
- Noncurrent financial liabilities = €5,689.9 million
- Noncurrent lease liabilities = €55.1 million
- Current financial liabilities = €650.1 million
- Current lease liabilities = €21.3 million

Total debt (financial liabilities + lease liabilities) = 5,689.9 + 55.1 + 650.1 + 21.3 = €6,416.4 million

Debt-to-equity ratio = 6,416.4 / 3,644.7 ≈ 1.76x, which indicates a moderately leveraged utility.

**2. S&P's 15% hybrid cap:**

S&P caps the equity credit from hybrid bonds at 15% of total adjusted capital (debt + equity).  
Total adjusted capital = 6,416.4 + 3,644.7 = €10,061.1 million  
15% cap = ~€1,509.2 million in hybrid bonds with equity credit.

**3. Compare with actual equity:**

Equity is €3,644.7 million, so €1,509.2 million in hybrids would be substantial but not overwhelming relative to reported equity.

**4. Interest rate environment (2022):**

The swap curves for 2022 show sharply higher rates:
- 5Y: 1.726% average
- 7Y: 1.806%
- 10Y: 1.927%

Corporate bond yields (iBoxx EUR Non-Financial IG sub-senior) averaged 2.295% in 2022. This means hybrid issuance would be relatively expensive compared to the low-rate environment of 2020–2021.

**5. Profitability and coverage:**

EBIT 2022 = €533.8 million  
Finance costs = €217.2 million  
Interest coverage = 533.8 / 217.2 ≈ 2.46x — adequate but not extremely strong.

**6. Utilities typically use hybrids:**

As a regulated utility with stable cash flows, HERA is a natural candidate for hybrid issuance to optimize capital structure without diluting equity. The company already has significant debt and could benefit from equity credit treatment.

**7. Recommendation:**

Given HERA's leverage profile, its likely desire to maintain investment-grade ratings while funding infrastructure investment, and the S&P cap, using a significant portion — but not all — of the hybrid capacity makes sense. Full 100% usage might be aggressive given rising rates and the existing debt load. A moderate utilization like 50% balances the benefits of equity credit with the higher cost of hybrid debt in the current rate environment.

<answer>50%</answer>