The increasing use of intermittent renewable energy sources to decarbonize electric power generation is expected to introduce dynamic instability to the mains. This situation is of particular concern for mini-grids or isolated grids in which wind and/or solar power sources are the dominant or the sole power sources. In this paper, we utilize the photo-electro-thermal theory to develop a design methodology for LED lighting systems for weakly regulated voltage sources, with the objectives of minimizing the fluctuation of the human luminous perception and adopting reliable LED driver with long lifetime and robustness against extreme weather conditions. The proposed LED system, practically verified in a 10 kVA small power grid driven by an ac voltage source and a wind energy simulator, can be considered as a smart load with its load demand following the power generation. A typical swing of 40 V in the mains will cause only 15% actual light variation in a 132 W LED system when compared with 40% change in 150 W high-pressure-sodium lamp system. The design methodology enables future large-scale LED systems to be designed as a new generation of smart loads that can adapt to the voltage and power fluctuations arising from the intermittent nature of renewable energy sources.