To determine the appropriate extent to which EDP, S.A.'s capital structure should rely on hybrid bonds, we must analyze the company's financial profile, industry characteristics, and the role hybrid bonds play in utility financing.

1. **Industry Profile & Credit Stability**: EDP is a major electric utility operating in the generation, transmission, distribution, and supply of electricity and gas. Utilities are characterized by highly stable, regulated cash flows, heavy capital expenditure requirements, and natural monopolies. This operational stability allows them to sustain significantly higher leverage ratios than typical non-financial corporates without facing distress.

2. **Current Capital Structure**: Looking at the 2022 balance sheet, EDP's total equity is €13.83 billion and total liabilities are €44.98 billion, resulting in a debt-to-equity ratio of over 3.2x (or ~76% debt). Total financial debt (long-term borrowings of €15.78bn + current borrowings of €4.24bn) stands at roughly €20 billion. Hybrid bonds typically qualify as 50% equity under IFRS accounting and receive 50% equity credit from rating agencies, allowing companies to raise leverage without proportionally impacting their credit ratings. 

3. **Optimal Reliance on Hybrids**: While hybrid bonds are an extremely useful tool for utilities to bridge the gap between high leverage and target credit metrics (such as maintaining an investment-grade rating of A- or BBB+), they are fundamentally subordinated, perpetual instruments with coupon step-up features, making them a permanent, higher-cost form of capital compared to senior debt. Relying on hybrids for 50%, 75%, or 100% of the capital structure is not commercially viable—such a structure would imply insufficient senior debt capacity (which contradicts the stable asset base of a utility) or an unsustainably high cost of capital. Utilities typically cap their hybrid bond usage at roughly 10-15% of total capitalization (equity + hybrids + debt) to optimize their weighted average cost of capital (WACC) and rating agency equity buffers. 

4. **Conclusion**: Given EDP's stable cash flows and massive asset base, it can comfortably support a large senior debt pile. Hybrid bonds should only be a marginal supplement to optimize the capital structure and support the firm's credit rating without exceeding the standard utility threshold of 15-20% of total equity/debt. Therefore, the reliance on hybrid bonds should be modest—effectively closer to a 0% baseline than any higher options, as senior debt and pure equity remain the overwhelming core of the capital structure. 

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