To determine the optimal extent to which EDP, S.A.'s capital structure should rely on hybrid bonds, we need to consider the relative costs and benefits of the available financing instruments. 

Hybrid bonds typically combine features of both debt and equity. While they offer certain advantages, such as providing a layer of equity-like cushion for credit rating agencies without diluting existing shareholders' equity (often treated as 50% equity / 50% debt by rating agencies), they come at a higher cost. As seen in the provided market data, the Subordinated-Senior (SUB-SEN) Delta is positive, meaning that the yield required by investors for hybrid/subordinated bonds is structurally higher than the yield required for senior debt. For instance, in 2022, the SUB-SEN delta was approximately 2.295%, reflecting the premium compensated for the subordinated risk.

From a pure Cost of Capital (WACC) optimization perspective:
1. **Senior Debt** is cheaper than hybrid debt and provides standard tax shields.
2. **Equity** is more expensive but provides ultimate financial flexibility and avoids distress costs.
3. **Hybrid Bonds** are more expensive than senior debt but do not provide the full flexibility of common equity. 

Unless a firm faces binding constraints regarding its credit rating and cannot issue standard senior debt or equity—which is not the case for a large, IG utility like EDP, S.A. with solid profitability (over €1.6B Profit Before Tax in 2022) and strong operating cash flows (€3.77B in 2022)—incorporating hybrid bonds into a theoretically optimal target capital structure simply increases the WACC compared to an optimal mix of pure senior debt and equity. Consequently, to minimize the overall cost of capital, the theoretical weight of hybrid bonds should be zero. 

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