The performance of solar photovoltaic (PV) systems has been widely investigated based on the low-frequency data such as hourly or daily solar data. However, its short-term performance monitored at high frequency has been relatively less studied. This paper presents the short-term performance of a grid-connected solar photovoltaic (PV) system monitored at 1-min interval from July 1, 2010 to June 30, 2011 and compares with the daily performance. The summary results based on mean, standard deviation, and coefficient of variation (CV) are presented. The results indicate that the yearly means of the minutely array output power and the minutely global horizontal irradiance (GHI) in a typical day are bell-shaped. The CV pattern of the minutely array output power appears to be U-shaped while the CV pattern of the minutely GHI is varying within a horizontal zone of [0, 5%]. More importantly, the minutely data reveal that the output power responds nonlinearly to changes in solar irradiance while the hourly data conventionally used suggest a linear relationship between them. A piecewise quadratic model was built to fit the nonlinear relationship between array output power and GHI based on the minutely data. The model was proved to be accurate with high value of R² and validated by the measured data.