Well, an accurate weather forecast is the first step to generate a good power production forecast. To have the most precise weather forecast, several weather models (NWP), are often used in a combination as one forecast calculation model might work best at high temperatures, while another in a certain region when winds are strong. The country and region determine the combination of models. Let us take Scotland for instance that has a complex terrain. The model mix here, are most likely different from the models used, to determine power generation for a power plant located in a flat terrain, in Denmark. So, weather models and the combination of models are important for precise prediction.
Yet, other elements are also of significance. Measurement data, location data and generation capacity from the power plant is an important component to predict future power generation in a given area. Here, the accuracy of the (historical) production data determine the precision of the power production forecasts. Information on data resolution, plant availability and potential curtailment also contributes to increase the forecast accuracy. Not to mention, metadata from individual turbines, which help determine any differences in power production on individual turbines, which again affect the entire power plant production.
So, data amount and accuracy, provide the basis to generate a good power production forecast!