To determine the optimal utilization of hybrid bonds relative to the 15% equity credit cap, we evaluate the company's financial profile as of the end of 2022.

1. **Capital Structure Analysis (2022-01-01):**
   - **Equity:** 4,894,276,000 EUR
   - **Debt (Noncurrent + Current Borrowings):** 5,491,124,000 (Long-term) + 721,845,000 (Short-term) = 6,212,969,000 EUR.
   - **Total Adjusted Capital (Debt + Equity):** 6,212,969,000 + 4,894,276,000 = 11,107,245,000 EUR.
   - **S&P 15% Cap:** 11,107,245,000 * 0.15 = 1,666,086,750 EUR.

2. **Cost of Debt and Market Environment:**
   - The company's finance costs in 2022 were 116,468,000 EUR on significant borrowings. With the Swap Curves (5Y, 7Y, 10Y) significantly increasing from 2021 to 2022 (e.g., 10Y Swap average moving from 0.053% to 1.927%), the cost of debt has risen considerably.
   - Hybrid bonds offer a mechanism to optimize credit ratings by receiving equity credit while maintaining control and avoiding dilution. Given that the company has a strong equity base and substantial capital expenditure requirements (reflected in "Purchase of Property Plant And Equipment" of 536,185,000 EUR), utilizing hybrid instruments up to the permitted regulatory/rating agency limit is a common financial strategy for utilities to manage leverage ratios while maintaining an investment-grade rating.

3. **Strategic Utilization:**
   - As a regulated utility (Red Eléctrica Corporación, S.A.), the company operates in a capital-intensive environment. Maintaining the 15% cap helps preserve credit metrics against interest rate volatility. Given the current market data and the company's financial position, there is no immediate indication of excessive leverage that would necessitate a lower threshold, and utilizing the full capacity provided by the rating agency is the standard approach to optimize the weighted average cost of capital (WACC).

Therefore, the company should utilize the available equity credit capacity fully.

<answer>100%</answer>