To determine the extent to which REN should utilize hybrid bonds relative to S&P Global Ratings' cap, we must evaluate the company's capital structure and its need for equity-like instruments to optimize its leverage metrics under S&P's criteria. 

1. **Calculate S&P's Total Adjusted Capital and the Hybrid Cap:**
S&P defines total adjusted capital as Debt + Equity. Hybrid bonds receive "equity credit" from S&P up to a maximum of 15% of this total adjusted capital.
- Total Debt (Long-term + Short-term Borrowings as of 2023-01-01) = 1,695,362,000 + 638,944,000 = 2,334,306,000 EUR
- Total Equity (as of 2023-01-01) = 1,517,534,000 EUR
- Total Adjusted Capital = Debt + Equity = 2,334,306,000 + 1,517,534,000 = 3,851,840,000 EUR
- S&P Maximum Hybrid Cap (15% of Total Adjusted Capital) = 577,776,000 EUR

2. **Assess the Capital Structure Context:**
REN is a regulated utility operating in the energy sector (electricity and natural gas networks). Regulated utilities inherently carry significant debt to finance large infrastructure projects. REN's Debt-to-Equity ratio is roughly 1.54x (2,334M / 1,518M), and its Debt-to-Capital ratio is ~60.6%. While not severely over-levered, its leverage is notably high for a regulated utility targeting solid investment-grade metrics, especially considering the massive spike in current assets/liabilities tied to the 2022 gas price stabilization regime, which introduces balance sheet volatility. 

3. **Determine the Optimal Utilization Level:**
Because S&P's equity credit for hybrids is limited to 15% of total adjusted capital, a company must decide how much of this cap to use. Issuing hybrids up to the cap (100% utilization) provides the maximum equity credit, lowering leverage ratios (like Debt-to-Equity) more effectively than pure equity without diluting existing shareholders. For a capital-intensive utility with a 60% debt-to-capital ratio, maximizing the use of the hybrid cap is a standard and highly efficient capital allocation strategy to manage S&P leverage metrics. The cap is inherently conservative (15%), meaning 100% utilization merely brings the effective equity floor to 15% of the total capital base, a level that aligns perfectly with maintaining an A-range rating for utilities. Partial utilization (25%, 50%, 75%) would unnecessarily forgo the leverage-reduction benefits that S&P explicitly permits, leaving the firm with sub-optimized metrics given its significant debt load.

Therefore, the company should fully utilize the permissible hybrid cap to optimally manage its leverage ratios and funding costs.

<answer>100%</answer>