In this paper, we demonstrate a new MEMS electrostatic power generator utilizing potassium ion electrets. The developed device with a 14.4 g proof mass exhibited 115 WRMS power generation by 0.03 GRMS acceleration. Using the developed power generator, we conducted a charging experiment under an acceleration waveform observed on a viaduct and established an electric energy of 4.8 mJ stored in a 44 F capacitor in 90 min.