To determine the optimal utilization of hybrid bonds relative to the 15% S&P equity credit cap, we evaluate Fortum’s financial position at the end of 2022:

1.  **Capital Structure Analysis**:
    *   **Equity (Total)**: 7,737,000,000 EUR.
    *   **Liabilities (Total)**: 15,905,000,000 EUR.
    *   **Total Adjusted Capital (Debt + Equity)**: 7,737,000,000 + 15,905,000,000 = 23,642,000,000 EUR.
    *   **15% Cap on Equity Credit**: 0.15 * 23,642,000,000 = 3,546,300,000 EUR.

2.  **Financial Context**:
    *   Fortum’s 2022 performance was significantly impacted by the loss from discontinued operations (-11,302,000,000 EUR) and a overall loss of -10,290,000,000 EUR.
    *   The company experienced a massive reduction in equity (from 13,665,000,000 EUR in 2022 to 7,737,000,000 EUR in 2023) and faced high volatility in derivative financial liabilities (16,657,000,000 EUR noncurrent and 71,947,000,000 EUR current in 2022, though significantly reduced by year-end 2023).
    *   Given the market interest rate environment in 2022 (Swap rates rising significantly from ~0% to ~1.8-1.9%) and the company's precarious liquidity and profit situation, maintaining flexibility while managing the credit rating is critical.

3.  **Strategy**:
    *   The company's high liability-to-equity ratio and the volatility of its capital structure suggest a need for caution. While hybrids provide equity-like characteristics, over-leveraging with hybrid debt could increase future interest burdens (finance income/cost was -193,000,000 EUR in 2022).
    *   Considering the credit cap is a hard limit, a conservative but efficient approach to maximize potential ratings support without over-extending financing costs suggests an intermediate position. 50% utilization of the allowable capacity represents a balanced approach to capital management in a period of high financial stress and interest rate uncertainty.

<answer>50%</answer>