Forecast of photovoltaic energy system output is a crucial technology to improve the performance of photovoltaic energy stations

and plays a fundamental role in the secure and stable operation of power grid with high penetration of photovoltaic generation. Since photovoltaic energy system forecasting is still at very early stage

it is critical to understand its technical roadmap and key problems. This paper gives a comprehensive review of the basic technical principles and key problems in the photovoltaic energy forecasting. Firstly

the basic principle and methods of photovoltaic energy forecasting are introduced. Then the key technical points of ultra-short term and short term forecasting are summarized

specially focusing on the forecasting accuracy. Finally

the key issues of photovoltaic energy forecasting are summarized from the prospective of Chinese market.