Solar cells' temperature is a very important parameter that affects performance of photovoltaic (PV) modules since main electrical parameters of PV cells and modules are temperature dependent regardless the technology. The present study evaluates and compares different sensor types and mountings for long term outdoor temperature monitoring of PV modules along with a standardized method for determination of cell's temperature from open-circuit voltage. For that purpose, a special multicrystalline silicon PV module with miniature in situ Pt1000 temperature sensors was used for reference temperature measurement. On the back side of the PV module different temperature sensors were attached, including thermocouple (TC), platinum Pt1000 (PT) and digital temperature sensors DS18B20 (DS). All sensors except one were covered by a 1 cm thick insulation block. The whole setup was mounted on the outdoor PV testing site and all temperatures were monitored for several days with selection of different environmental conditions. On the basis of measurement results, deviations of different temperature sensors are investigated and compared to temperature calculated from open-circuit voltage measurement according to standard EN 60904-5. Among sensors attached at the back side, covered PT and TC sensors deliver the best results in range of 1–2 °C of lower temperature in average; while the covered DS sensor gives additional 1–2 °C underestimated temperature values. The worst measurement results demonstrate the PT sensor without insulation. All temperature sensors exhibit similar and adequate time response regarding the thermal capacitance of the PV module. DS sensors, although showing somewhat worse results, offer great advantages if several temperatures have to be acquired simultaneously and require very simple data acquisition equipment. They feature comparable measurement accuracy than commonly used Pt1000 temperature sensors if they are covered by insulation with 10 mm thick walls in lateral direction to avoid micro-environmental changes.