In recent years, the CubeSat project has become a useful tool to teach university students about space engineering. Getting started with the CubeSat is still a barrier to educational institutions that are new to space technology. Having a satellite training kit to teach students about satellite is an effective approach. This paper provides a design process of the Electrical Power Subsystem (EPS) of the educational satellite training tool in the form of 1U CubeSat called DemoSat. After the components and sensors are to be included in the DemoSat are defined, power system architecture are designed, and power consumption is estimated. Based on requirements of the EPS, the suitable hardware components are then selected. The EPS hardware uses commercially available off-the-shelf (COTS) components including DC adapter, battery charger, boost converter and the Lithium-ion battery. To make it user-friendly for beginners, the DemoSat hardware is designed based on the through-hole components. For the energy storage, the DemoSat only employed one cell of Lithium-ion battery and it can be operated for approximately four hours, which is considered enough for teaching in the classroom. The total hardware cost is less than 200 USD which is affordable for university students. The EPS of DemoSat has almost all of the EPS components found in a normal CubeSat, except for the solar array input which we planned to include in the next version.