To determine the extent to which A2A S.p.A. should utilize hybrid bonds relative to the 15% equity credit cap, we evaluate the company's financial position and the market environment provided:

1.  **Capital Structure Analysis:** Based on the 2022 year-end report, total equity is 4,467,000,000 EUR and total liabilities are 16,900,000,000 EUR. The total adjusted capital (Debt + Equity) is 21,367,000,000 EUR. 15% of this total is 3,205,050,000 EUR. This represents the maximum "equity credit" allowed by S&P.
2.  **Market Environment:** The interest rate environment for 2022 showed a significant shift, with 5Y, 7Y, and 10Y swap curves rising from near-zero/negative territory in 2021 to approximately 1.7-1.9% in 2022. Similarly, credit spreads (IBOXX EUR Non-Financial IG) widened from 1.298% to 2.295%.
3.  **Strategic Rationale:** Hybrid bonds provide equity-like characteristics (often 50% equity credit) while being classified as debt, helping to maintain credit ratings without diluting existing shareholders. Given the rise in the cost of debt (increasing swap rates and wider credit spreads), the issuance of new hybrid instruments becomes more expensive but potentially more attractive compared to common equity issuance if the firm seeks to optimize its weighted average cost of capital (WACC) while supporting its investment-grade rating.
4.  **Capacity:** With equity at ~4.47B EUR and significant non-current liabilities (7.2B EUR), the company has ample room to increase leverage. Utilizing hybrids up to the full 15% limit of total adjusted capital provides the maximum possible support to the capital base under S&P's criteria. Given the company's large-scale operations (Revenue of ~23B EUR) and its expansion/investment requirements, maximizing this efficient capital tool is a standard corporate finance approach.

Therefore, the company should utilize the hybrid bond capacity fully (100% of the allowable limit) to achieve optimal capital efficiency.

<answer>100%</answer>