To determine the optimal utilization of hybrid bonds relative to the 15% S&P Global Ratings' cap, we must analyze the company's financial structure and interest rate environment.

1. **Financial Position**: EDP, S.A. reported "Equity" of 13,834,608,000 EUR and "Liabilities" of 44,981,803,000 EUR for 2023. Total adjusted capital (debt + equity) is approximately 58.8 billion EUR. The 15% cap on equity credit equates to approximately 8.8 billion EUR in hybrid issuance eligibility.
2. **Interest Rate Environment**: Market data shows a significant shift in swap rates (5Y, 7Y, 10Y) between 2021 and 2022. The 5Y swap rate rose from an average of -0.264% in 2021 to 1.726% in 2022, and the 10Y swap rate rose from 0.053% to 1.927%. This indicates a rising interest rate environment, increasing the cost of issuing new traditional debt and making the equity-like features of hybrid bonds more attractive to support the credit rating.
3. **Strategic Need**: With a high level of liabilities relative to equity (a debt-to-equity ratio > 3x), the company is incentivized to maintain or improve its credit metrics to reduce the cost of capital. Utilizing the full limit of the S&P equity credit (15% of adjusted capital) provides the most significant support for the balance sheet, allowing the company to maintain leverage ratios within acceptable bounds despite rising interest expenses (Finance Costs rose from 875M EUR to 1.75B EUR).

Given the significant increase in financial costs and the need to optimize the capital structure in a rising rate environment, the company should maximize its use of instruments that receive equity credit to maintain its financial profile. Therefore, utilizing the full extent of the permitted hybrid equity credit (100% of the S&P cap) is the most prudent strategy to preserve creditworthiness.

<answer>100%</answer>