ABSTRACT This paper addresses how to apply model‐based systems engineering (MBSE) to ultrasmall planar satellites under severe power and resource constraints. PlanarSats are sub‐100‐gram spacecraft built as single printed‐circuit boards where the same surface must host both electronics and solar cells, so power generation, geometry, and component placement are tightly coupled. Conventional MBSE with a full requirements–functional–logical–physical (RFLP) stack is often too heavy for the small, fast teams that build these missions. We propose a tailored MBSE framework that keeps compact requirements and functional view, but makes a combined Physical/Electrical view the central driver. This view uses a power‐first sizing methodology based on current best estimate, maximum expected value, and maximum possible value of power, and derives an operational power envelope that links surface area, mode power, and allowable Sun incidence angles. Operational and verification views are reduced to a simple power‐aware state machine and a small requirement‐to‐test matrix. The framework is demonstrated on a conceptual sub‐100‐gram, batteryless radiation‐sensing PlanarSat in low Earth orbit, showing that the design remains power‐positive in all planned sunlit modes while using a reduced set of MBSE artefacts maintained in a lightweight, tool‐supported modeling environment. The approach is intended to help small teams retain traceability and explicit constraints for PlanarSat missions without the overhead of enterprise‐scale MBSE deployment. The framework is proposed and evaluated analytically on this conceptual case study; no hardware or flight data are presented.