To assess the extent to which ENEL S.p.A. should utilize hybrid bonds over the next 18 months, we must evaluate the company's current capital structure, its operational and financial trajectory, and the explicit issuance constraints provided.

**1. Current Capital Structure & Total Adjusted Capital (TAC)**
As of December 31, 2022 (2023-01-01 in the report), ENEL's financial position is as follows:
*   **Equity:** €42.08 billion
*   **Short-term Borrowings:** €18.39 billion
*   **Long-term Borrowings:** €68.19 billion
*   **Total Debt:** €86.58 billion
*   **Current Outstanding Hybrids:** €5.567 billion (reported under "Equity Instruments Perpetual Hybrid Bonds Member")

Using a gross approach, ENEL's Total Adjusted Capital (TAC) is approximately **€128.66 billion** (Equity + Total Debt). 
At present, hybrids account for about **4.3%** of its TAC (€5.567B / €128.66B).

**2. Capex Intensity and Leverage Pressure**
ENEL requires significant ongoing investments to fund its energy transition and grid updates. In 2022, the company spent €11.28 billion on PP&E and €1.96 billion on intangible assets, yielding a massive capital expenditure burden of over €13 billion. Additionally, ENEL paid €4.9 billion in dividends. 
While operating cash flow was robust at €8.67 billion, it was insufficient to cover these massive outflows, resulting in significant negative free cash flow. This forced ENEL to rely heavily on external financing, leading to a massive increase in total debt from €67.8 billion in 2021 to €86.58 billion in 2022. Due to this significant leverage pressure and constrained rating headroom, relying on hybrid bonds to protect its investment-grade rating and optimize its capital structure is highly advisable. 

**3. Issuance Constraints and Mathematical Feasibility**
While ENEL's aggressive capex and leverage profile qualitatively echo the traits of higher brackets, we must factor in the mathematical constraint that the company can issue a maximum of **€3 billion in hybrids per year**.
*   Over the next **18 months**, the maximum amount of new hybrid bonds ENEL can issue is **€4.5 billion**.
*   Adding this to the current €5.567 billion brings the absolute maximum hybrid capacity to **~€10.07 billion**.
*   Against a TAC of €128.66 billion, a €10.07 billion hybrid layer represents approximately **7.8% of TAC**.

If ENEL were to target the 11.25% bracket, it would need roughly €14.47 billion in hybrids, requiring nearly €9 billion in new issuance over 18 months—well beyond the €4.5 billion limit. Targeting the **7.5%** bracket requires a total hybrid layer of roughly €9.65 billion. This translates to exactly €4.08 billion in new issuance over the next 18 months (or ~€2.72 billion per year), which perfectly aligns with the €3 billion annual capacity constraint. 

**4. Market Conditions**
With 5Y and 7Y swap curves jumping to ~1.7%-1.8% in 2022 and the sub-senior delta expanding, the cost of issuing hybrids has climbed. Thus, issuing hybrids will moderately increase the current cost of debt, aligning with the 7.5% criteria.

**Conclusion**
Given the mathematical cap on near-term issuance, coupled with the need to stabilize leverage amid an intense capex program, targeting a 7.5% hybrid ratio is the optimal, realistic ceiling for the next 18 months.

<answer>7.5%</answer>