To determine the appropriate level of hybrid bond issuance for ENEL - SPA over the next 18 months, we evaluate the company's current capital structure, financial metrics, and market environment against the provided guidance. 

1. **Current Capital Structure & Outstanding Hybrids:**
As of the end of 2022, Enel's Total Equity was roughly €42 billion. Its total reported debt (short-term + long-term) was approximately €86.5 billion, and with cash equivalents of €11 billion, its Adjusted Debt (Net Debt) is about €75.5 billion. This places Enel's Total Adjusted Capital (TAC) around €117.6 billion. 
The company's balance sheet already reports €5.56 billion under "Equity Instruments Perpetual Hybrid Bonds," which constitutes roughly 4.7% of its TAC. Advising a target of 3.75% would imply net redemptions, contradicting the goal of maintaining or optimizing their current hybrid stack to support funding. 

2. **Leverage & Rating Headroom:**
Enel's S&P Net Debt to EBITDA ratio stands at a moderately elevated 4.84x, and its FFO to Net Debt ratio is 15.67%. These metrics indicate that while Enel retains a solid investment-grade profile, its rating headroom is moderately constrained. A hybrid bucket of 7.5% allows the company to defend its rating metrics efficiently without putting extreme pressure on its cost of capital. Additionally, Moody’s recognized the company’s adjusted leverage trend as "Improving," indicating that a drastic pivot to maximum hybrid utilization (11.25% or 15%) is unwarranted and unnecessary.

3. **Capital Expenditure & Refinancing Needs:**
Enel exhibits considerable capital intensity—recording over €11.2 billion in PPE purchases in 2022, indicative of a large energy transition and grid modernization pipeline. This creates moderate to high refinancing needs. However, issuing around €3 billion per year (reaching a stock of ~€8.5 billion, or ~7.5% of TAC) fits perfectly within the reasonable issuance limits to comfortably bridge funding gaps without saturating their investor base.

4. **Cost of Debt:**
Market data from 2022 shows a substantial spike in swap rates and corporate bond yields compared to 2021. As borrowing costs have risen, the incremental cost of subordinated hybrid debt will slightly increase Enel's overall cost of capital, making the 7.5% tier the optimal balance between cost-efficiency and equity credit.

By raising its hybrid stock toward the 7.5% threshold, Enel can fund its significant capex program, meaningfully improve its adjusted leverage, and preserve its credit rating headroom without over-relying on expensive subordinated instruments.

<answer>7.5%</answer>