This paper conducts a comprehensive comparative analysis of multiple input multiple output (MIMO) based rectenna configurations for energy harvesting in ultra-low power applications. Utilising simulation-based methods, various MIMO topologies and different antenna numbers within each topology are assessed to gauge their efficiency and output power, with mathematical models devised to quantify their performance accurately. The findings indicate a corresponding rise in output power with an increased number of antennas in the MIMO configuration. The superior output power of the DC combining topology is particularly noteworthy, achieving levels of up to 300 mW at 20 dBm with 16 antennas, surpassing that of the RF combining topology. Nevertheless, the RF combining topology demonstrates outstanding power performance when operating with low input RF power. Initially, the voltage doubler rectifier used in the study exhibited an efficiency of 51%. However, the DC combining MIMO rectenna achieves a remarkable efficiency of 98%. Overall, the study underscores the superiority of the DC combining topology for RF energy harvesting applications, offering valuable insights for enhancing indoor device energy efficiency and fostering the development of sustainable wireless systems amidst advancing technologies.