To determine the appropriate extent of hybrid bond usage in A2A Energia S.p.A.'s capital structure, we must evaluate the company's financial stability, leverage, and the cost of debt environment.

1.  **Financial Leverage and Liquidity:** The company’s total assets grew from 18.0 billion EUR to 21.4 billion EUR, while total liabilities increased from 13.7 billion EUR to 16.9 billion EUR. The Debt-to-Equity ratio remains relatively high, and the company has significant capital expenditure requirements (Property, Plant, and Equipment increased by ~0.6 billion EUR, and intangible assets by ~0.4 billion EUR).
2.  **Market Conditions:** The market data shows a sharp increase in swap rates and bond yields in 2022. The 5Y swap rate moved from -0.264% in 2021 to 1.726% in 2022, and the 10Y swap rate from 0.053% to 1.927%. Furthermore, the iBoxx EUR Non-Financial IG spread (delta-adjusted) increased significantly (from 1.298% to 2.295%). 
3.  **Rationale for Hybrids:** Hybrid bonds offer an efficient way to improve credit metrics (as they are often treated partially as equity by rating agencies) while managing the interest burden. Given the sharp rise in borrowing costs (Finance Costs increased from 89 million EUR in 2021 to 125 million EUR in 2022), relying on hybrid capital helps mitigate the risk of credit rating downgrades in a high-interest-rate environment.
4.  **Conservative Approach:** While hybrid instruments are beneficial, they are more expensive than traditional senior debt. A 100% or 75% reliance would create excessive interest burdens and potential dividend-like obligations that could hinder flexibility. A 50% reliance is also likely too high for a standard utility entity, which typically prefers a mix heavily weighted toward senior secured/unsecured debt for tax efficiency and cost. However, given the current need to maintain financial strength while facing rising market rates, a 25% allocation provides a balanced approach to supporting the capital structure without incurring the high costs associated with excessive hybrid issuance.

<answer>25%</answer>