To determine the appropriate extent to which Terna S.p.A.'s capital structure should rely on hybrid bonds, we must consider the company's financial characteristics, industry norms, and the specific role hybrid bonds play in corporate finance.

1. **Company and Industry Profile:** Terna is the primary electricity transmission system operator in Italy. This makes it a regulated utility with highly stable, predictable cash flows and a natural monopoly. Such companies typically carry high levels of debt due to the capital-intensive nature of their infrastructure, which generates reliable returns. 

2. **Current Capital Structure and Hybrid Usage:** Looking at the 2022 balance sheet, Terna's total equity is €6.17 billion, and total liabilities are €22.8 billion. The debt component (long-term borrowings, current portion of long-term borrowings, and short-term borrowings) amounts to roughly €10.76 billion. Notably, Terna already utilizes hybrid bonds, with €989 million classified under "Capital Instruments Bonds Hybrid Perpetual" within equity. This accounts for roughly 16% of their total equity and around 6-7% of their total long-term capitalization.

3. **Role of Hybrid Bonds:** Hybrid bonds are treated as 50% equity by rating agencies (like Fitch and S&P) up to a certain limit (typically around 25-30% of total adjusted capital). They provide equity credit, which supports the company's credit rating and keeps leverage ratios manageable, while still offering the tax deductibility of interest payments like traditional debt. For a utility that needs to fund continuous heavy capital expenditures (CAPEX) without diluting shareholders or overleveraging, hybrids are an ideal tool.

4. **Assessing the Appropriate Limit:** 
- **0%** is too conservative; it ignores the tax and rating agency benefits hybrids provide, especially given Terna's stable utility cash flows which can easily service hybrid coupons.
- **50% or more** is excessive. Relying on hybrids for half the capital structure would push the limits of equity credit from rating agencies, significantly increase refinancing risk (as hybrids often have step-up features and call dates), and subordinately position creditors, making traditional debt unnecessarily expensive.
- **25%** represents the industry best practice and the standard ceiling for equity credit recognition. It allows a regulated utility like Terna to optimize its weighted average cost of capital (WACC), maintain a solid investment-grade rating by bolstering equity metrics, and efficiently fund its massive transmission grid investments. It aligns with the upper bound of where utilities typically cap their hybrid issuance to retain favorable accounting and rating treatment.

Given Terna's existing ~7% hybrid utilization, there is ample room to increase this to optimize capital structure, but it should be capped around the 25% threshold of total capital to maintain rating agency equity treatment and financial stability.

<answer>25%</answer>