The increasing penetration of variable renewable energy sources, such as wind and solar photovoltaic (PV), requires these technologies to actively support power system operation through participation in day-ahead and balancing markets, including the provision of ancillary services. In this context, hybrid power plants, where wind and solar PV share a common point of common coupling and are operated as a single unit, can provide technical and economic advantages compared to standalone plants. This work focuses on the “firm capacity” that can be offered in the Portuguese balancing markets using probabilistic power forecasts, considering the hybridisation of existing wind power plants with solar PV units. Results are compared with the standalone wind power plant and feature-based indicators are also explored to assess whether dynamic selection of forecast quantiles can increase the available capacity for balancing market. Two case studies in Portugal with different levels of wind and solar complementarity are considered. Results show that hybrid power plants significantly increase the amount of capacity available for market participation from 20.5% to 66.2% relative to standalone wind power plants. Thus, hybridisation enhances the ability of existing grid assets to provide ancillary services through participation in balancing markets.