This thesis presents an open-source, ultra-low powered data-logger for off-grid photovoltaic (PV) applications. An off-grid PV energy system is also designed for a rural house in Pakistan. The real-life power consumption data of this house is collected for the design and simulation purpose. The expected annual output energy of designed system is calculated by using Homer Pro software. Annual solar irradiance, average temperature and other environmental aspects are also considered for simulation of the designed system in Homer Pro. The data-logger is designed to log major parameters of designed PV energy system. Deep-sleep mode of ESP32-S2 microcontroller is used along with voltage, current, and light sensors for logging the data of PV system in an external micro SD card. Data-logger is programmed to operate in deep-sleep and web-portal monitoring modes and a manual or automatic switch is used to select these modes. Real-time PV data can be monitored in a local web-portal programmed in the microcontroller only by switching the toggle switch to on position. The same web-portal is also used to check and download the historical data of a PV system. The energy consumption of the designed system is 7.33mWh during deep-sleep mode and 425mWh during the web-portal monitoring mode. The total cost of the designed data-logger is approximately 30 CAD.