To determine the optimal utilization of hybrid bonds for TenneT Holding B.V., we first calculate the Total Adjusted Capital (TAC) as of 2022-01-01 (the end of the reporting period).

1.  **Equity and Debt figures:**
    *   Equity (2023-01-01): 7,713,000,000 EUR
    *   Longterm Borrowings (2023-01-01): 19,006,000,000 EUR
    *   Shortterm Borrowings (2023-01-01): 709,000,000 EUR
    *   Hybrid Capital (2023-01-01): 2,125,000,000 EUR

2.  **Adjusted Capital Calculation:**
    S&P methodology for hybrid capacity typically defines Total Adjusted Capital as Debt + Equity.
    *   Debt (Long + Short) = 19,006,000,000 + 709,000,000 = 19,715,000,000 EUR.
    *   Equity = 7,713,000,000 EUR.
    *   Total Capital = 19,715,000,000 + 7,713,000,000 = 27,428,000,000 EUR.

3.  **Hybrid Cap Analysis:**
    *   The S&P cap is 15% of Total Adjusted Capital.
    *   15% of 27,428,000,000 = 4,114,200,000 EUR.
    *   TenneT currently has 2,125,000,000 EUR in Hybrid Capital.

4.  **Strategic Context:**
    TenneT is a transmission system operator (TSO) with a massive capital expenditure requirement (4,424,000,000 EUR in 2022). With current interest rates rising significantly (2022 Swap curves ~1.7%-1.9% vs ~0% in 2021) and the company operating at a loss, maintaining credit rating headroom is critical. They have significant room under the 15% S&P cap (4.11B vs 2.12B). However, issuing to the full limit is often considered aggressive. Given the infrastructure nature of the business and the need to preserve credit metrics while funding major transition projects, a 50% utilization of the remaining capacity relative to the total threshold is a balanced approach.

<answer>50%</answer>