To determine the optimal utilization of hybrid bonds relative to the 15% S&P Global Ratings equity credit cap, we must analyze Terna S.p.A.'s financial structure at the end of 2022.

1.  **Total Adjusted Capital Calculation**:
    *   **Equity**: The total Equity reported for 2023-01-01 is 6,169,100,000 EUR.
    *   **Debt**: Total Borrowings (Short-term + Long-term) = 444,100,000 + 8,416,700,000 = 8,860,800,000 EUR.
    *   **Current Portion of Long-term Borrowings**: 1,909,300,000 EUR.
    *   Total Debt ≈ 10,770,100,000 EUR.
    *   Total Adjusted Capital = Debt + Equity = 10,770,100,000 + 6,169,100,000 = 16,939,200,000 EUR.

2.  **S&P Equity Credit Cap**:
    *   The cap is 15% of total adjusted capital: 0.15 * 16,939,200,000 = 2,540,880,000 EUR.

3.  **Current Hybrid Utilization**:
    *   The company lists "Capital Instruments Bonds Hybrid Perpetual" at 989,000,000 EUR as of 2023-01-01.
    *   The existing hybrid bond volume (989 million EUR) is well below the 15% cap (2.54 billion EUR).

4.  **Strategic Context**:
    *   Terna is a utility company with a stable revenue profile (Regulated assets).
    *   Interest rates in 2022 saw a significant increase (Swap curves shifted from negative territory to roughly 1.7%-1.9%).
    *   Given the significant gap between existing hybrid debt and the maximum allowed equity credit, there is room to issue additional hybrid instruments to optimize capital structure and support infrastructure investments while staying within credit rating guidelines. However, in corporate finance, moving from the current level toward the cap is typically done incrementally. Given the choices, 25% represents an appropriate level of the available capacity headroom to utilize, balancing financial flexibility with the benefit of equity credit.

<answer>25%</answer>