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• Day-ahead and intraday photovoltaic (PV) forecast accuracy is analyzed for 17 ENTSOE members. • Intraday forecasts are less skillful than day-ahead, signing a huge potential for improvement. • 4 TWh positive and 3 TWh negative annual regulatory needs from PV forecast errors for Europe. • Energy storage is required to ensure the carbon-free grid integration of PV power plants. As photovoltaic (PV) electricity generation is becoming ubiquitous, the ability to forecast solar power becomes crucial for such aspects as economical dispatch, optimal unit commitment and the stability of the grid. Our research investigated the discrepancies between day-ahead and intraday country-specific PV power generation forecasts and the real generation data in the member states of the European Network of Transmission System Operators for Electricity for the year 2021. The goal of the research was, firstly, to offer an insight into the schedule accuracy achieved in the examined countries, secondly, to present the potentials for improvement in each of them (plausible even by the practical application of the methods available in 2021), and thirdly, to show the amount of regulation need resulting from PV electricity production forecasting error, the knowledge of which can help with the estimation of the size of the necessary storage capacities. This is the first work that analyzes the schedule accuracy from a practical perspective by examining the accuracy of the day-ahead and intraday PV forecasts of seventeen European countries, following the latest recommendations for verification. The results found significant differences between the forecast accuracy of different countries, which indicates a huge potential for imbalance reduction in most countries by improving their own algorithms or creating a uniform European PV power generation forecasting system. Overall, the intraday forecasts are less skillful than the day-ahead forecasts in all but one of the countries, which highlights the significance of further application-related studies in the intraday horizon. The aggregate regulatory need resulting from the discrepancies between the forecasted and actually produced amounts of energy of all studied countries is 4 TWh in the positive and 3 TWh in the negative direction, which calls for a large amount of energy storage capacity to provide carbon-free balancing power and reduce the regulation challenges of the increasing PV penetration. [ABSTRACT FROM AUTHOR]