Abstract Improvement of the quantitative performance characterization of photovoltaic (PV) strings was investigated, based on their monitoring data during maximum power point tracking (MPPT) operation. The maximum power voltage Vmp and current Imp of the PV strings under MPPT were continuously monitored, and corrected to standard temperature of 25 °C by using the temperature correction formulas, which were recently developed for PV modules. The irradiance G was measured by using a PV module irradiance sensor. It was verified that the formulas are applicable to PV strings, enabling reproducible temperature correction of Vmp and Imp with relative standard deviation (σ) of about 0.4–0.9% under various weather conditions. Comparison of the outdoor results with indoor current–voltage curve measurements of the constituent modules in the strings showed that the maximum output power Pmax under the standard test conditions STC, i.e. 25 °C and 1 kWm−2, estimated from the temperature-corrected Vmp and Imp, agreed with the indoor result typically within ±1.5%. The experimental results also showed that other information of the string such as partial shading and activation of bypass diodes can be also sensitively detected by analyzing the temperature-corrected Vmp and Imp. The outdoor measurements and data analysis of this study can be carried out without stopping the MPPT operation of the string. The analysis is performed by using the real-time data. The present method is expected to be applicable to most kinds of crystalline silicon PV strings without modification.