To determine the optimal reliance on hybrid bonds for ERG S.p.A.'s capital structure, we must evaluate the company's profile and the relative costs of different debt instruments.

1. **Company Profile**: ERG S.p.A. is a non-financial corporate entity operating primarily in the renewable energy sector (wind and solar). Unlike financial institutions (such as banks or insurance companies), non-financial companies are not subject to stringent regulatory capital requirements (e.g., Basel III) that necessitate a specific proportion of Tier 1 or Tier 2 hybrid capital. 

2. **Cost of Capital**: The provided market data highlights the yields for IG corporate bonds and the "SUB-SEN DELTA FOR IBOXX EUR NON-FINANCIAL IG". This delta represents the premium paid on subordinated/hybrid debt over senior debt. For instance, in 2022, the average yield for standard corporate bonds was 1.085%, while the subordinated/hybrid debt incurred a substantial premium (a total average yield of 2.295%). 

3. **Capital Structure Optimization**: From a pure cost-of-capital perspective, hybrid bonds are structurally subordinated and therefore carry a higher yield than senior unsecured debt. Because ERG S.p.A. has steady operating cash flows (EBITDA of ~€499M in 2022) to support traditional senior debt and project financing, utilizing more expensive hybrid bonds would unnecessarily increase its Weighted Average Cost of Capital (WACC). Without regulatory requirements forcing the issuance of hybrids, the rational choice to minimize financing costs is to rely on standard senior debt and equity.

Therefore, the company should not rely on hybrid bonds for its optimal capital structure.

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