To determine the appropriate level of hybrid bond issuance for Fortum Oyj, we must analyze its financial profile, credit metrics, and future needs based on the provided 2022 annual report data and the S&P rating methodology guidelines.

**1. Financial Profile and Leverage Context**
Fortum's 2022 financials are heavily impacted by discontinued operations (the Russian Uniper segment), resulting in a massive net loss of -€10.29 billion. However, to assess the going-concern credit profile and hybrid capacity, we look at the post-restructuring balance sheet as of Dec 31, 2022:
- Total Equity: €7.737 billion
- Total Liabilities: €15.905 billion (of which €4.127bn is other current financial liabilities, €3.973bn is current derivatives, €3.658bn is other non-current financial liabilities, and €0.756bn is non-current derivatives). Total debt-like financial liabilities amount to roughly €12.5 billion.
- Total Adjusted Capital (Equity + Adjusted Debt) is approximately €20.2 billion.

**2. Rating Headroom and Leverage Pressure**
The massive losses and impairments in 2022 have severely degraded Fortum's retained earnings and overall equity, significantly weakening its leverage metrics (Debt-to-Equity and FFO-to-Debt). The company went from a robust equity of €13.665 billion in 2021 to €7.737 billion in 2022. This represents severe leverage pressure and constrained rating headroom, threatening its investment-grade profile. Under S&P methodology for Regulated/Unregulated Utilities, stabilizing the leverage trajectory is critical to preserving the rating.

**3. Refinancing and Capex Needs**
Fortum has substantial refinancing needs. Its short-term financial liabilities (current financial liabilities plus current derivatives) stand at over €8 billion. While operating cash flows from continuing operations remain strong (€2.1 billion), the sheer scale of near-term maturities and the need to fund future capex—especially for the green energy transition and potential Nordic nuclear investments—indicates very high funding needs. 

**4. Hybrid Rationale and Cost**
S&P limits equity credit for hybrids to a maximum of 15% of total adjusted capital. For a company with €20.2 billion in adjusted capital, 15% equates to roughly €3 billion. Given:
- Very high refinancing needs;
- Material downgrade risk without capital structure optimization after the Uniper collapse;
- Hybrids substantially helping to stabilize the adjusted leverage trajectory by bolstering the equity credit;

Hybrid bonds are a highly attractive tool for Fortum. The cost of hybrid debt in 2022 (swap rates + sub-senior delta) is roughly 2.3% to 2.5%. While this is higher than the average cost of Fortum's legacy debt (implied by its €179m interest expense on larger pre-2022 debt stacks), it is not materially penalizing given the steep increase in benchmark rates. More importantly, the marginal cost of hybrids is justified by the substantial rating support they provide (equity credit), which ultimately lowers the overall cost of capital by preserving the investment-grade rating. Fortum already has an established history with hybrids in its capital structure, making the 15% threshold fully accessible.

Because Fortum faces material downgrade risk without equity support, has very high refinancing requirements, and requires substantial capital structure optimization to recover from the 2022 shock, it should utilize the maximum allowable equity credit for hybrids under S&P's criteria.

<answer>15%</answer>