Based on the financial data and characteristics of ACEA S.P.A., the company is advised to maintain a 0% allocation of hybrid bonds as a percentage of total adjusted capital over the next 18 months. 

Here is the reasoning:
1. **Historical Issuance:** The company has never issued hybrid bonds in the past ("First year of hybrid bond issuance: never" and "Issued hybrid bonds in 2021 or 2022: no"). An entity's past capital structure choices are a very strong indicator of its future funding strategy. The absence of current hybrid bonds in the capital structure strongly points to a 0% recommendation.
2. **Stable Utility Profile:** As a multi-utility operating largely in regulated sectors (water, electricity grids, public lighting), ACEA S.P.A. typically benefits from a highly predictable and transparent regulatory framework. This stability usually allows it to maintain a robust financial risk profile and access long-term debt capital easily without the need for expensive equity-like hybrid instruments.
3. **Cost of Capital:** In the rising interest rate environment observed in 2022 (with the 5Y and 10Y swap curves shifting up materially), issuing hybrid debt for the first time would carry a significant subordination premium. Introducing hybrids now would materially increase the company's average cost of debt without necessarily bringing much-needed rating relief, as there are no imminent transformational M&A or extraordinary capex pressures flagged that would jeopardize its credit metrics.
4. **Credit Metrics and Leverage Optimization:** The company's underlying operating cash flows are steady. The lack of any historical reliance on hybrids suggests its investment-grade rating and leverage metrics are comfortable without the equity credit that hybrid bonds provide. 

Consequently, staying away from hybrids (0%) is the most prudent and logical capital structure decision.

<answer>0%</answer>