To determine to what extent ERG S.p.A. should utilize hybrid bonds relative to S&P Global Ratings' cap, we evaluate the cost dynamics between different sources of capital. 

Hybrid bonds are subordinate to senior debt but rank senior to common equity. Because they sit higher in the capital structure than equity, investors demand a lower rate of return for hybrids compared to the cost of equity (i.e., Cost of Equity > Cost of Hybrid > Cost of Debt). Furthermore, hybrid bond coupons are often tax-deductible, providing an additional tax shield that lowers the firm's Weighted Average Cost of Capital (WACC), a benefit that common equity does not share. 

S&P Global Ratings allows companies to classify hybrid bonds as 50% equity and 50% debt, providing an "equity credit" up to a cap of 15% of the total adjusted capital (Total Debt + Equity). Since hybrid debt is strictly cheaper than common equity, any firm looking to optimize its capital structure and minimize its WACC should theoretically replace the maximum allowable amount of equity with hybrid bonds. 

Because replacing equity with hybrids reduces the overall cost of capital without losing the regulatory/rating benefits of equity (up to the limit), a cost-minimizing company should max out this allowance. Therefore, the optimal utilization of the hybrid bond cap is its absolute limit.

<answer>100%</answer>