To determine the appropriate reliance on hybrid bonds for Fortum Oyj's capital structure, we must consider the company's financial profile, industry characteristics, and the specific role hybrid bonds play in corporate finance.

1. **Industry Characteristics**: Fortum is a major Nordic energy company with significant operations in nuclear and hydro power, as well as district heating. Such capital-intensive, regulated, and strategically important utility businesses typically carry high fixed-asset bases and require stable, long-term financing. Utilities traditionally employ conservative capital structures to maintain investment-grade credit ratings and ensure financial stability.

2. **Current Capital Structure**: As of the end of 2022, Fortum's Total Equity was 7,737 million EUR, and Total Liabilities were 15,905 million EUR, resulting in Total Capital of 23,642 million EUR. The Debt-to-Capital ratio (assuming all financial liabilities function as debt) is approximately 67%. Even if we net out non-debt liabilities like trade payables and provisions, the proportion of traditional debt in the capital structure is substantial, but the equity base remains significant.

3. **Hybrid Bond Functionality**: Hybrid bonds (such as subordinated notes) possess both debt and equity characteristics. They are treated as equity by rating agencies (usually receiving 50% equity credit) due to their deep subordination, long tenors, and optional deferral of interest payments, which provides financial flexibility. However, they are treated as debt for tax purposes, allowing the company to benefit from the tax shield on interest payments. 

4. **Optimal Reliance**: While hybrid bonds are an excellent tool for optimizing the weighted average cost of capital (WACC) and achieving equity desired by rating agencies without diluting shareholders, they come with higher coupon rates compared to senior debt. Over-reliance on them (e.g., 50% or more) would disproportionately increase the cost of capital and could signal financial distress or aggressive leverage, potentially negating the rating agency equity credit. An allocation of 0% fails to capitalize on the tax benefits and equity-like flexibility that hybrids offer to a capital-intensive utility. A 25% weighting in the capital structure is a common and prudent level for European investment-grade utilities—enough to meaningfully optimize WACC, secure partial equity credit from agencies, and provide a buffer during volatile times (like the 2022 energy crisis), without crowding out traditional senior debt or becoming excessively expensive.

Given Fortum's need to maintain a conservative, investment-grade profile typical of European utilities while optimizing its capital structure for cost and flexibility, a 25% reliance on hybrid bonds is the most appropriate and common industry practice.

<answer>25%</answer>