This article details the design, modeling, construction, and evaluation of an open system calorimeter that operates in a normal room environment to measure endothermic or exothermic events in a system subjected to a steady heat flux. The calorimeter is unique because it allows the measurement of energy and power from an "open" system, where a heat flux enters and leaves the calorimetric boundary in a well-controlled manner. It is also novel because it utilizes a solid state heating and cooling assembly that acts as an electronic heat reservoir. The system is capable of measuring power levels from a few milliwatts to several watts, and it has been designed and optimized to be nearly immune to variations at ambient temperature and room airflow. The calorimeter was modeled using lumped parameter electrical-thermal equivalent circuits in SPICE software. This modeling in the electrical domain led to the use of a mathematical correction factor that mitigates mismatches in thermal conduction paths between an active and a passive cell as well as correcting differences in the sensitivities of the flux sensors employed for heat flow measurement. After obtaining a viable design, a prototype was constructed and validated with precise input power delivered via electric joule heating of a resistive element.