To determine the appropriate amount of hybrid bonds for Enel S.p.A., we must assess its current financial position, credit metrics, and future funding needs against the S&P methodology and provided guidelines.

**1. Current Capital Structure & Leverage:**
- Total Equity (2022): €42,082 million
- Total Financial Debt: Summing long-term borrowings (€68,191M), short-term borrowings (€18,392M), and current portion of long-term borrowings (€2,835M) gives €89,418 million.
- Total Adjusted Capital (Equity + Debt) ≈ €131,500 million.
- Net Debt / EBITDA: EBITDA can be approximated as Operating Profit (€11,193M) + Depreciation & Amortization (€7,447M) = €18,640 million. Net Debt (approx. €89,418M - €11,041M cash = €78,377M) / EBITDA results in roughly 4.2x. For a regulated utility under S&P methodology, a leverage ratio above 4.0x typically falls into the "Significant" financial risk profile (BBB range), implying limited rating headroom and potential downgrade risk.

**2. Existing Hybrids:**
- Enel already has €5,567 million in "Equity Instruments Perpetual Hybrid Bonds". This represents approximately 4.2% of its total adjusted capital. S&P limits equity credit for hybrids to a maximum of 15% of total adjusted capital, leaving roughly 10.8% (or ~€14 billion) of headroom under the cap. The maximum issuance per year is capped at €3 billion.

**3. Funding Needs & Capex Intensity:**
- Enel exhibits high capital intensity. In 2022, Purchase of Property, Plant, and Equipment alone was €11,281 million, with another €1,961 million for intangibles.
- The company has significant refinancing needs, with over €21 billion in short-term and current long-term debt maturing.
- Despite the operating profit increase in 2022, net income fell drastically (from €3.857B to €2.92B) due to massive losses from discontinued operations (-€2.298B) and rising interest rates. Cash flow from operations (€8.674B) was insufficient to cover total capex (~€14.5B including subsidiaries/business combinations), resulting in a heavy reliance on debt financing (Proceeds from borrowings were €22.4B).

**4. Interest Rate Environment & Cost of Debt:**
- In 2022, the ECB aggressively hiked rates, pushing the 5Y Swap curve from -0.264% (2021) to 1.726% (2022). Corporate spreads (iBoxx Euro Corp Bond) widened from 0.733% to 1.085%.
- While Enel's average cost of debt was historically low, new debt and hybrids will be priced at considerably higher yields. However, for a utility with significant leverage pressure needing to preserve its Investment Grade rating, the incremental cost of hybrids (which offer 50% equity credit from S&P) is marginally justified to stabilize the leverage trajectory.

**5. Assessment against Guidelines:**
- **0%**: Incorrect. Enel has high capex/refinancing needs and leverage pressure, and already uses hybrids.
- **3.75%**: Too conservative given the high capex and need for structural leverage optimization to protect the IG rating. 
- **7.5%**: Plausible, but may not sufficiently offset the leverage trajectory given the scale of the capex program and current ~4.2x leverage.
- **11.25%**: Highly appropriate. Enel faces high capex intensity and significant leverage pressure. Preserving its Investment Grade profile (A-/BBB+) is strongly dependent on capital structure optimization. Issuing up to €3 billion in 2023 (bringing the total to ~€8.5B, or ~6.5% of capital) and continuing into 2024 aligns with utilizing the 11.25% threshold over the next 18 months to meaningfully reduce adjusted leverage metrics. The cost of hybrids, while higher than 2021, is necessary to secure the 50% equity credit required to stabilize Net Debt/EBITDA.
- **15%**: Excessive at this stage. Enel's leverage is pressured but not at the point of material downgrade risk requiring the absolute maximum equity credit allowable. It has other deleveraging tools (asset disposals, which are already underway as seen by the discontinued operations).

Therefore, targeting the 11.25% range allows Enel to effectively manage its high capex and refinancing needs, providing essential rating headroom without excessively diluting its cost of capital.

<answer>11.25%</answer>