To determine the appropriate extent to which Acciona SA's capital structure should rely on hybrid bonds, we must consider the company's financial profile, the nature of its business, and the specific characteristics of hybrid bonds.

1. **Business Model and Asset Base**: Acciona is a large infrastructure and renewable energy conglomerate. Such businesses are heavily capital-intensive, requiring massive upfront investment in property, plant, and equipment (€9.485 billion in 2022) and long-term projects. These assets generate stable, predictable, and long-duration cash flows (operating cash flow of €1.648 billion in 2022). This profile naturally supports a higher degree of leverage.

2. **Current Capital Structure**: Looking at the 2022 balance sheet, Acciona has a highly leveraged capital structure. Total Equity is €6.304 billion, while Total Liabilities stand at €16.291 billion. Financial debt (current and noncurrent loans and obligations) totals roughly €6.416 billion. The Debt-to-Equity ratio is notably high (over 1.0x for financial debt alone, and much higher including total liabilities). 

3. **Hybrid Bond Characteristics**: Hybrid bonds possess both debt and equity characteristics. From a rating agency perspective (e.g., S&P, Moody's, Fitch), they are often treated as "equity credit" (usually 50% to 100% depending on features like deferral options and perpetual nature), which helps alleviate leverage ratios. However, they carry higher coupon rates than senior debt due to subordination, and excessive reliance on them can lead to steeply escalating financing costs that drag down net income.

4. **Optimal Reliance**: While hybrid bonds are useful for a leveraged capital-intensive company like Acciona to optimize its weighted average cost of capital (WACC), manage covenant headroom, and achieve intermediate equity treatment without diluting shareholders, they cannot replace traditional debt or equity entirely. The high coupon burden makes them inefficient as a primary funding source. A moderate allocation—typically around 25% of the total capital structure (or up to a quarter of the total equity-equivalent base)—is optimal for infrastructure firms to gain the rating agency equity cushion and balance sheet flexibility without suffering prohibitive financing costs or excessive subordination risk. Anything above this (50%, 75%, 100%) would be financially unsustainable and drastically increase the cost of capital, while 0% ignores a valuable tool for a highly leveraged firm.

<answer>25%</answer>