This study analyzes renewable energy forecasting accuracy and associated imbalance costs across Baltic electricity markets using ENTSO-E transparency data and proprietary plant-level datasets. The research reveals significant variations in specific imbalance costs between countries, with some showing costs aligned with European literature while others exhibit anomalously high values, likely due to data quality issues in transparency platform reporting. Plant-level analysis demonstrates that while solar forecasts achieve better accuracy metrics, they incur higher imbalance costs due to systematic over- forecasting bias. Portfolio aggregation effects show clear benefits, with combined assets achieving reduced forecast errors and lower imbalance costs compared to individual plants. The findings underscore the critical importance of forecast accuracy improvements, portfolio diversification strategies, and standardized data reporting protocols for effective renewable energy market participation.