CubeSats have become a popular satellite-standard choice for space missions due to their versatility and affordability. Despite their widespread use, extensive testing is still necessary to minimize in-orbit failures. In this paper, the design of a solar simulator, primarily for evaluating the energy management of CubeSats, is presented. The simulator consists of a box with LEDs on its internal walls, where a CubeSat is placed inside. By controlling the LEDs, it is possible to emulate the in-orbit conditions in which the CubeSat operates, allowing its power generation capacity to be assessed. Though the tests are conducted under non-space conditions, the controlled environment makes it possible to identify problems in CubeSat's power management, as well as to analyse the operation of its Electric Power System (EPS). The solar simulator can evaluate transient energy generation as well as battery performance by simulating varied orbits. It can also expose the CubeSat to critical scenarios to test its behavior under these conditions. Then, this paper outlines the design of the solar simulator and its components, such as the selection of LEDs and heatsinks. It also presents a preliminary methodology for controlling LEDs aiming to replicate in-orbit conditions. The solar simulator is currently under development, but it is expected to aid in the design and testing of CubeSats. This includes evaluating individual components like photovoltaic panels and batteries, as well as subsystems such as the EPS, ultimately benefiting the overall design of the space segment.