To determine the extent to which A2A S.p.A. should rely on hybrid bonds in its capital structure, we must evaluate its current financial health, leverage, and the cost of debt.

1. **Leverage and Debt Capacity**: A2A's Net Debt can be approximated by taking its total financial liabilities and subtracting cash and equivalents. With noncurrent and current financial liabilities at approximately €6.89 billion and cash and equivalents at €2.58 billion, the Net Debt is roughly €4.31 billion. Given its 2022 Gross Operating Income (EBITDA) of €1.50 billion, its Net Debt/EBITDA ratio is approximately 2.8x. This is a very comfortable leverage ratio for a regulated utility company, comfortably supporting an investment-grade credit rating.
2. **Interest Coverage**: With an EBIT (Profit from Operating Activities) of €687 million and Finance Costs of €125 million, its interest coverage ratio is strong (around 5.5x), meaning the company generates more than enough operating income to easily service its existing debt obligations.
3. **Cost of Capital**: The provided market data indicates a significant spread (Sub-Sen Delta) for subordinated/hybrid debt over senior investment-grade debt, jumping to nearly 2.30% in 2022. 

Because A2A maintains robust debt capacity, strong interest coverage, and a solid financial profile, it can readily access standard senior debt at much lower interest rates. Hybrid bonds—which are subordinated and carry higher yields to compensate investors for added risk—are generally utilized by companies needing to defend their credit ratings from a downgrade or those nearing their maximum debt capacity. Since A2A is not exhibiting signs of financial distress or over-leverage, the inclusion of more expensive hybrid bonds would sub-optimally increase its Weighted Average Cost of Capital (WACC) without providing necessary structural benefits.

Therefore, the optimal reliance on hybrid bonds for A2A S.p.A.'s capital structure under these circumstances is 0%.

<answer>0%</answer>