To determine the appropriate extent to which ERG S.p.A.'s capital structure should rely on hybrid bonds, we must evaluate the company's financial profile, industry characteristics, and the strategic role of hybrid capital.

1. **Industry Profile & Capital Intensity**: ERG S.p.A. is a major renewable energy producer (wind, solar, hydro) with highly capital-intensive operations. Its balance sheet shows substantial Property, Plant, and Equipment (€2.12 billion) and Service Concession Rights (€0.96 billion). This asset-heavy profile requires long-term, stable financing to match the long-term nature of its concession agreements and power generation assets. 

2. **Current Financial Health & Leverage**: Looking at the 2022 year-end figures, ERG's Total Equity is €2.05 billion and Total Liabilities are €3.17 billion, yielding a Debt-to-Equity ratio of approximately 1.54x. Net Debt (Total Financial Liabilities of roughly €1.75B non-current + €0.39B current financial liabilities minus €0.39B cash) stands around €1.75 billion, resulting in a Net Debt to Equity of roughly 0.85x. This is a moderate and healthy leverage level for a utility, showing that the firm is not under-leveraged nor in financial distress.

3. **Role of Hybrid Bonds**: Hybrid bonds (such as subordinated corporate hybrid bonds) are treated as 50% equity by rating agencies (e.g., S&P, Fitch) due to their deeply subordinated nature, long-dated maturity (often perpetual or with 50+ year duration), and optional coupon deferral. They are primarily used by utilities and infrastructure companies to:
   - Optimize their credit rating by artificially lowering leverage metrics (allowing them to hold more senior secured debt against predictable cash flows).
   - Fund long-lived assets without diluting common equity.
   - Provide balance sheet flexibility for M&A (ERG had significant M&A activity in 2022, as seen by the increase in Service Concession Rights, Goodwill, and cash outflows for obtaining control of subsidiaries).

4. **Extent of Reliance**: Utilities typically target hybrid bonds to constitute around 10% to 15% of their total capitalization (or up to 25% of their equity base) to achieve the optimal rating agency "equity credit" without excessively increasing the overall cost of capital (since hybrid coupons are notably higher than senior debt). A 25% reliance of the capital structure on hybrid bonds would imply hybrid debt equals roughly a third of the firm's total equity, which is the standard ceiling for maximizing rating agency equity treatment. A 50% or greater reliance would be excessively costly, disproportionately subordinate senior creditors, and defeat the purpose of capital structure optimization. A 0% reliance ignores the distinct tax and rating advantages that renewable infrastructure companies actively leverage. 

Given ERG's strong but predictable cash-generating asset base, moderate current leverage, and active expansion strategy, a 25% reliance on hybrid bonds within its capital structure aligns perfectly with standard utility finance practices for credit optimization and long-term asset matching.

<answer>25%</answer>