Based on the provided annual report data for ENEL SPA for the fiscal year ended December 31, 2022, and the guidelines for hybrid bond issuance, my assessment is as follows.

First, I note the entity's history with hybrid bonds. The data shows that "Equity Instruments Perpetual Hybrid Bonds Member" had a balance of €2,386 million at the start of 2021, €5,567 million at the end of 2021 (and start of 2022), and €5,567 million at the end of 2022. The entity issued €3,181 million in hybrid bonds during 2021 and €0 during 2022. The entity has a significant and active history of using hybrid bonds, but did not issue any in the most recent fiscal year.

Now, I evaluate the entity's financial profile:
- **Profitability and Leverage:** The S&P Net Debt/EBITDA ratio for 2022 is 4.84x, and the S&P FFO/Net Debt is 0.1567 (15.67%). These metrics suggest a leverage profile consistent with the "BBB" to "BB" range, indicating moderate to significant leverage pressure, not a strong "A" range profile.
- **Funding and Capital Expenditure:** The cash flow statement shows net operating cash flow of €8,674 million, which is strong. However, capital expenditure is very high, with purchases of property, plant, and equipment of €11,281 million. Net cash used in investing activities is €13,626 million. This indicates a high capex intensity that is not fully covered by operating cash flow, leading to a need for external financing.
- **Financing Activities:** The entity had significant financing activities, with proceeds from borrowings of €22,399 million and repayments of €9,359 million, indicating large refinancing needs and ongoing debt management.
- **Cost of Debt:** The market data shows a sharp rise in the swap curve (e.g., 10Y swap from 0.053% in 2021 to 1.927% in 2022) and corporate bond yields. The cost of new debt, including hybrids, has materially increased. The subordinated debt spread (Sub-Sen Delta) also increased, meaning the all-in cost of a new hybrid would be significantly higher than in prior years.

Weighing these factors against the guidelines:
- The entity has a **high capex intensity** and significant funding needs, as shown by the large negative free operating cash flow before capex and the high volume of financing flows.
- The credit metrics indicate significant **leverage pressure**, with a Net Debt/EBITDA of 4.84x, making investment grade preservation a key focus.
- The entity has a demonstrated history of using hybrid bonds to manage its capital structure, with a current outstanding balance of over €5.5 billion. This signals that it views hybrids as a viable tool, but the election not to issue any in 2022, a year of rising rates, is also a signal.
- Issuing a large amount of new hybrids in the next 18 months, when the cost of debt has risen sharply, would conflict with the guideline that at the 11.25%-15% levels, the cost should have a "marginal" to "in-line" impact on current cost of debt. The current market data indicates a new hybrid would materially increase the cost of debt.
- Given the current adjusted capital, a new issuance of up to €3 billion would fall between the 3.75% and 7.5% thresholds. The guideline for 3.75% mentions "mild leverage optimization" and a cost that "will increase the current cost of debt," while 7.5% mentions "moderate refinancing or acquisition needs" and a cost that "will slightly increase the current cost of debt."

Considering the significant refinancing needs, high capex cycle, and balance sheet pressure, but also the materially higher market rates and the entity's pause on issuance in 2022, a moderate level of hybrid issuance seems most advisable. The entity needs to optimize its leverage, but cost sensitivity is elevated. A 7.5% level would provide meaningful improvement to adjusted leverage metrics for a moderately constrained rating headroom, while the cost would "slightly increase" the current cost of debt, which is a more accurate description than "marginal" given the rate environment.

Therefore, the advised level for hybrid bond issuance in the next 18 months is 7.5%.

<answer>7.5%</answer>