I. INTRODUCTION UE to the rapid increase in deployment and high penetration of solar power in electricity grids worldwide, solar power generation forecasting has become critical to variable generation integration planning and within utility and ISO operations. State-of-the-art solar

Due to the rapid increase in deployment and high penetration of solar power generation worldwide, solar power generation forecasting has become critical to variable generation integration planning, and within utility and independent system operator (ISO) operations. Utilities and ISOs require day ahead and hour ahead as well as intra-hour solar power forecasts for core operations solar power producers and energy traders also require high quality solar power forecasts. As a result of the erroneously perceived simplicity of solar radiation forecasting, very often non-repeatable, poorly explained or obscure estimates of solar power forecast performance are used. This creates uncertainty with the quality of forecasting service, as well as unrealistic expectations of possible forecast precision. As a result, there is an immediate need for defining a common methodology for evaluating forecast performance, establishing verification procedures, and setting common standards for industry-approved quality of solar forecast performance. Solar power forecast quality claims can be easily verified when the source of forecast is known. Most often the offered power generation forecasts are based on publically available results of Numerical Weather Prediction (NWP) models and on the use of empirical relationships between solar resource and generated power at a specific plant. The quality of these forecasts is limited by the quality of the NWP models utilized, which is known. Less frequently, solar radiation is estimated based on proprietary models such as satellite-based or total sky imager–based cloud cover and radiation forecasts. In such cases, there are also known limits to the accuracy of prediction which can help objectively evaluate claims of the forecast service companies. This paper is proposing a set of standards for evaluating intra hour, hour ahead, day ahead and week ahead solar power forecast performance. The proposed standards are based on sound methodologies and extensive field practice and offer a solid ground for reliable inter-agency comparisons of forecast performance. Index Terms — Forecast standards, RMSE, bias, persistence, NWP, performance evaluation.