To determine the optimal extent to which A2A S.p.A. should utilize hybrid bonds relative to S&P Global Ratings' cap, we must analyze the impact of hybrid bonds on the company's Weighted Average Cost of Capital (WACC). 

The goal of utilizing hybrid bonds up to the S&P limit (15% of total adjusted capital, which receives 50% equity credit) is to replace more expensive equity with a relatively cheaper source of financing, without negatively affecting the company's credit rating.

1. **Cost of Debt ($K_d$)**: Can be approximated using the iShares Core Euro Corp Bond average for 2022, which is 1.085%.
2. **Cost of Hybrid ($K_h$)**: Hybrid bonds typically price at a spread (sub-sen delta) over senior debt. Using the 2022 Sub-Sen Delta for the iBoxx EUR Non-Financial IG index (2.295%), the estimated cost of hybrid debt is roughly $1.085\% + 2.295\% = 3.38\%$.
3. **Cost of Equity ($K_e$)**: The 10Y Swap Curve (risk-free rate proxy) for 2022 is 1.927%. Factoring in a standard market risk premium (e.g., 5-6%) and a typical utility/energy beta, the cost of equity for A2A would reasonably be estimated in the 6.0% - 8.0% range. 

Because hybrid bonds are treated as 50% equity and 50% debt by S&P, issuing hybrid bonds allows the company to replace a mix of pure equity and senior debt. The after-tax cost of hybrid bonds (approx. 2.57% assuming a standard corporate tax rate of 24%) is significantly lower than the cost of pure equity (approx. >6.0%). 

Mathematically, since $K_e > K_h > K_d$, replacing equity with hybrid bonds strictly decreases the firm's WACC linearly. Because this is a linear relationship, the firm minimizes its WACC by substituting as much equity with hybrid debt as the rating agencies allow before the firm loses the 50% equity credit treatment. 

Therefore, to minimize its WACC while maximizing its cost efficiency without deteriorating its credit profile, A2A S.p.A. should fully utilize its hybrid allowance.

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