To determine the extent to which Acea S.p.A. should utilize hybrid bonds relative to S&P Global Ratings' 15% cap, we must evaluate the company's financial profile, including its capital structure, cash flow generation, and the prevailing interest rate environment.

1. **Capital Structure & Leverage**: Based on the 2022 balance sheet, Acea's total equity is €2,755,243,000, and its total debt (financial liabilities) is €5,655,681,000 (€5,451,487,000 noncurrent + €204,194,000 current). This results in a Debt/Equity ratio of approximately 2.05x. For a utility company, this leverage is moderate but on the higher side. S&P's 15% cap means the maximum equity credit from hybrids cannot exceed 15% of Total Adjusted Capital (Debt + Equity). Here, 15% of total capital amounts to roughly €1,261,638,000 (or €413,286,450 in absolute equity credit if treated as 50% equity). The company currently has negligible hybrid capital, giving it ample room to issue hybrids to optimize its capital structure and lower its effective leverage without hitting the cap. Thus, from a capacity standpoint, they should utilize a significant portion of the cap.

2. **Cash Flow Generation**: Utilities are characterized by stable, regulated cash flows. Acea's operating cash flow (OCF) remained robust at €726,703,000 in 2022 despite inflationary pressures. The OCF to total debt ratio is around 12.8%, which is somewhat low for an investment-grade utility, meaning they need to carefully manage debt servicing. 

3. **Interest Rate Environment**: The swap curve data indicates that interest rates rose sharply in 2022 (5Y swap went from -0.264% in 2021 to 1.726% in 2022). Similarly, corporate bond yields (iBoxx Euro Corp Bond) increased from 0.733% in 2021 to 1.085% in 2022. Sub-senior spreads also widened, indicating higher borrowing costs for subordinated/hybrid debt. Because of this rising rate environment and widened spreads, issuing long-dated hybrid bonds at the bottom of the current rate cycle (or during a "bear" scenario of 2.49% sub-sen yields) is less attractive than it was in 2021. However, for a utility looking to fund massive capex (Acea saw significant increases in intangible and tangible asset investments in 2022), hybrid bonds remain a crucial tool for maintaining credit rating metrics without diluting equity.

4. **Optimal Utilization**: Rating agencies like S&P typically view hybrid utilization favorably up to a "moderate" level, often interpreted as around half of the allowable cap, to ensure there is sufficient cushion and that the equity-like nature of the hybrids is genuinely supporting the capital structure during stress. Pushing too close to the 15% cap (75-100% utilization) can sometimes draw scrutiny regarding the true permanence of the capital, especially if cash flows face regulatory headwinds. Conversely, utilizing only 0-25% leaves cheap, equity-creditable capital on the table. A 50% utilization strikes the right balance, optimizing the cost of capital and leverage metrics while maintaining a conservative buffer below the 15% threshold, which is prudent given the rising rate and spread environment observed in 2022.

<answer>50%</answer>