Due to limitations in performance of thermoelectric materials, applying two-stage thermoelectric generator (TTEG) has been proposed to improve the performance of thermoelectric generator (TEG) system. In this paper, a novel prototype of a two-stage thermoelectric generator system is investigated experimentally. In the first stage, a TEG module installed between a phase change material (PCM) heat sink, as cooling system, and an electrical heater, as the heat source. Because of the inherent characteristics of PCMs to save the thermal energy as latent heat, the PCM heat sink is used as the heat source of the second stage TEGs. In the second stage, five smaller TEG modules are installed around the PCM with individual heat sinks for cooling with natural convection. In order to have a comparison between a common TEG system and the proposed two-stage TEG system, a one-stage thermoelectric generator with forced air cooling system has been tested. The results show the proposed TTEG system averagely generates 27% more electrical potential than the one-stage TEG system. Moreover, when the heater is off, the TTEG supplies 0.377 V open circuit voltage in average for about 7900 s, while the one-stage TEG generates this amount of voltage just for 2100 s. Therefore, the proposed design makes TEG systems more suitable for wireless sensor applications when the heat source does not provide steady thermal energy. In this study, four different patterns of thermal power applied to the TTEG system are considered. These patterns are used to simulate various transient thermal boundary conditions imposed to the system.