The Power Subsystem generates and distributes electrical power. The Solar Panels are the only source of electrical power generation on the satellite, thus they’re crucial for its operation. The more surface area covered by these solar cells, the more potential for power generation. However, there are additional components that need to occupy the surface area of the CubeSat, such as the transceiver, antenna and the optical payload.
The Battery is also critical for satellite operation. The component power usage can overtax the panel generation, thus they must be able to draw on the battery.