A light-emitting diode, also known as an LED, is a modern lighting technology that consumes 70% to 85% less energy than other lighting technologies. However, an LED is a nonlinear load that generates harmonic distortion, thereby distorting the sinusoidal waveforms. A larger number of LED installations in buildings may lead to severe power quality issues, and there is a lack of research on LED harmonic monitoring and laboratory testing, with underdeveloped innovation in practical power harmonic monitoring of LED lamps. To monitor the harmonic level in the system, the study aimed to design and develop a low-cost power-harmonic monitoring device for LED systems. The comparison of the Fluke 1736 power logger with a prototype using a low-cost ESP32 microcontroller with current and voltage sensors involved an experimental setup comprising a single LED and 18 LED tubes operated in a low-voltage, also known as an LV network, single-phase, 60Hz. The Total Harmonic Distortion or THD measurement method was applied, and the harmonic results were validated against specific IEC standards. It was observed that a single FSL T8 18W LED had a 236.3V and a 0.14A, with 0.51 pf, 94.7% total current harmonic distortion (THDi), and 1.5% total voltage harmonic distortion (THDv). For the 18 LED tube orientation, it produced 242.7V and 2.2A, with 0.56 pf, 133.8% THDi, and 1.7% THDv. The absolute error of the prototype is ±0.5V and ±0.02A, with THDi and THDv at ±1.35% and ±0.04%, respectively, which are within acceptable error limits of the IEC standards for low-cost instrumentation Class II and S. The findings showed that the FSL T8 18W LED has a poor power factor with a high THD content. The current harmonic distortion is directly related to the number of LED installations. The study revealed that it is essential to monitor power harmonics in modern lighting technology, and this can be achieved with a low-cost power-harmonic monitoring device.