The variability of wind and solar photovoltaic (PV) generation poses significant risks for producers in day-ahead electricity markets, where commitments must be made before actual output is realized. A common mitigation strategy is to invest in storage, but an alternative is to exploit the natural complementarity between wind and solar resources. We evaluate this economic value of cooperation using two stochastic optimization models with chance-constrained reliability guarantees: a Naive model without storage and a Storage-Enhanced model with battery dynamics. We evaluate the models using the European Meteorological derived High Resolution RES (EMHIRES) dataset for hourly wind and PV generation in 28 European countries in 2015. Our results show that cooperation consistently improves outcomes. Without storage, 24 of 28 countries achieve at least a 20% increase in expected profit, with gains in Finland, Lithuania, and Poland exceeding a seven-fold improvement. With storage included, absolute profits rise, but the relative advantage of cooperation becomes smaller and more uniform: joint profits are typically within 10–50% of the best independent operation. Cooperation remains most beneficial in countries like Belgium, Bulgaria, and the Netherlands, where wind and solar resources are both substantial and weakly negatively correlated. Methodologically, our work provides a demonstration of how chance-constrained optimization can evaluate renewable cooperation. For policymakers, our findings suggest that encouraging cooperative bidding can yield substantial and stable benefits in regions with balanced resources, whereas in resource-scarce settings, expanding capacity or scaling storage should take precedence.