To determine the appropriate extent to which VERBUND AG's capital structure should rely on hybrid bonds, we must evaluate the company's financial profile, business model, and the strategic role hybrid bonds play.

1. **Business Model and Risk Profile**: VERBUND is Austria's leading electricity utility, heavily reliant on hydroelectric power (as indicated by its massive Property, Plant, and Equipment base of nearly €11.9 billion). Utilities with stable, regulated, or quasi-regulated cash flows and high asset bases typically maintain high leverage because their predictable earnings can safely service debt. 
2. **Current Capital Structure**: As of the end of 2022, the company's total assets were €19.16 billion, with equity at €8.32 billion and total liabilities at €10.83 billion. This translates to a debt-to-equity ratio of approximately 1.3x (or 43% Debt-to-Total-Capital). While this is a conservative leverage ratio for a utility, it is standard for investment-grade energy companies.
3. **Interest Coverage and Cash Flows**: VERBUND is highly profitable, with an EBITDA of €3.16 billion in 2022 and strong free cash flow. The interest expense was a mere €101.7 million, yielding an exceptionally high interest coverage ratio (over 30x). This indicates the company has immense capacity to take on senior debt at lower costs without threatening its investment-grade rating.
4. **Role of Hybrid Bonds**: Hybrid bonds are treated as 50% equity by rating agencies (like S&P and Fitch) for up to a certain limit (usually the "intermediate equity" ceiling, which is typically around 25% of total adjusted capital for utilities). They are used by companies that want to maintain a specific rating while pushing leverage higher, or that need to fund large capex without diluting shareholders.
5. **Optimal Reliance**: For a utility like VERBUND, relying on hybrid bonds up to the rating agencies' equity credit limit is standard practice to optimize the weighted average cost of capital (WACC). Since hybrids are more expensive than senior debt (reflected in the higher corporate bond yields and swap spreads in the provided market data), issuing them beyond the ~25% threshold that qualifies for equity credit is inefficient—it unnecessarily increases the cost of capital without providing additional rating agency benefits. Given VERBUND's low current leverage, they could add hybrids to reach this sweet spot, making 25% of the capital structure the optimal reliance limit. 0% would be suboptimal for WACC, while 50% or more would be cost-prohibitive and exceed standard rating agency limits for equity credit.

<answer>25%</answer>