We evaluate the performance of TDM (Time Division Multiplexing)-based wireless power transfer (WPT) systems with simulation experiments in indoor time variant channels and investigate a TDM scheduling method using user grouping to achieve efficient and effective WPT. In the simulation experiments, the time-variant channels are generated by randomly moving objects that represent walking people. The TDM scheduler periodically switches multiple optimized weight vectors for MISO (Multiple-Input Single-Output) energy transfer at each time slot. We use the grouping strategy that we have already proposed to classify receivers according to the spatial correlation of the channel. The simulation results verify that the proposed grouping strategy can improve energy transfer efficiency even in the time-variant channel. We also discuss the possibility of the dynamic grouping method that switches user grouping according to the instantaneous channel state.