To determine the optimal utilization of hybrid bonds for REN relative to S&P Global Ratings' 15% cap on total adjusted capital, we must evaluate the cost of capital dynamics. 

1. **Cost Comparison**: Hybrid bonds generally have a higher coupon rate than senior unsecured debt (due to subordination and longer maturities) but are significantly cheaper than the cost of equity. 
2. **Equity Credit**: S&P Global Ratings allows up to 15% of a company's total adjusted capital (debt + equity) to be composed of hybrid bonds while granting them 50% equity credit. This characteristic is valuable because it allows the company to protect its credit rating metrics (like FFO/Debt and Debt/EBITDA) while raising capital that is cheaper than common equity.
3. **WACC Optimization**: In a traditional Weighted Average Cost of Capital (WACC) optimization model, as long as the after-tax cost of hybrid debt remains strictly less than the cost of equity, the model will prioritize replacing expensive equity with cheaper hybrid capital. 

Because replacing equity with hybrid debt lowers the overall WACC without violating the rating agency's threshold for equity credit, an optimal capital structure strategy will maximize this advantage. Therefore, the company should utilize its hybrid capacity up to the absolute maximum allowable limit.

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