Public & Social Infrastructure · January 2026 · 8 min read
Sizing the first-loss tranche: the number that unlocks blended finance
Blended finance is often described as a mix of concessional and commercial capital. That framing is why so many deals never close. The concessional tranche is not a mix; it is a specific number that sits at a specific place in the waterfall, and that number is set by the stochastic downside of the cash-flow model.
The argument
A first-loss tranche has to be sized to absorb the P20 shortfall of the base-case cash flow across the specific risks the commercial layer refuses to carry — FX devaluation, regulatory-delay-driven tariff shortfall, demand ramp underperformance. Anything smaller and the commercial tranche does not clear its risk committee. Anything larger and the concessional provider cannot justify the subsidy. The right number is a Monte Carlo output, not a negotiation anchor.
What we see in the field
On a recent East African broadband tariff, sizing the first-loss tranche to the P20 FX-and-delay shortfall closed a two-year structuring gap in a single conversation. The number came out of the model. It could not have come out of the term-sheet negotiation, which is why the negotiation had stalled.
What it changes
For DFIs, the first-loss tranche is a modelled output, not a policy allocation. For sponsors, the blended stack is only ready to market once the concessional layer is sized to a level the commercial layer accepts.
Where to start
Before approaching any concessional provider, produce the cash-flow Monte Carlo and identify the scenario at which the commercial layer clears. Size the first-loss tranche to that number and take it into the room as the anchor.
