To evaluate the extent to which ERG S.p.A.'s capital structure should rely on hybrid bonds, we consider the company's financial health, interest rate environment, and leverage profile based on the 2022 annual report data.

1.  **Financial Position:** ERG S.p.A. shows a strong profit performance, with a 2022 net profit of 383,098,000 EUR. The company has successfully managed significant cash flows from the disposal of businesses (1.26 billion EUR).
2.  **Solvency and Leverage:** The company has reduced its total liabilities significantly from 4.43 billion EUR in 2022 to 3.17 billion EUR in 2023. Its equity has increased from 1.57 billion EUR to 2.05 billion EUR, reflecting a deleveraging trend.
3.  **Interest Rate Environment:** The market data shows a sharp increase in swap rates across the board (e.g., 10Y swap rate moving from 0.053 in 2021 to 1.927 in 2022). Hybrid bonds (subordinated debt) generally carry higher coupons than senior debt, and given the rising cost of debt, issuing new hybrid instruments is expensive.
4.  **Capital Structure Strategy:** Given that ERG is already deleveraging and has healthy cash reserves (392 million EUR), a high reliance on expensive hybrid capital (which is often used to optimize credit ratings for companies with high leverage) is less optimal than maintaining a more traditional balance between senior debt and equity. A moderate exposure to hybrid instruments is often appropriate to maintain flexibility without incurring the excessive costs associated with pure hybrid reliance in a high-rate environment.

Considering the company's current financial strength and the shift in interest rates, a 25% reliance on hybrid bonds is appropriate to support capital structure efficiency without over-leveraging the cost of capital.

<answer>25%</answer>