This paper develops new real time prediction models for output power and energy efficiency of solar photovoltaic(PV) systemsThese models were validated using measured data of a grid-connected solar PV system in MacauBoth yearly and monthly averaged time frames are consideredIt is shown that the prediction model for the yearly/monthly average of the minutely output power fits the measured data very well with high value of R squareThe online prediction model for system efficiency is based on the ratio of the predicted output power to the predicted solar irradianceThis ratio model is shown to be able to fit the intermediate phase very well but not accurate for the growth and decay phases where the system efficiency is near zeroHowever, it can still serve as a useful purpose for practitioners as most PV systems work in the most efficient manner over this periodIt is shown that the maximum monthly average minutely efficiency varies over a small range of 10.81%-12.63% in different months with slightly higher efficiency in winter months.