Supercapacitors-based storage systems are expected to play a key role in microgrids in view of their capability to compensate high-power imbalances. We define an agent for the control of supercapacitor arrays within the context of the novel control framework Commelec, proposed by the Authors as a composable method for real-time control of active distribution networks with explicit power setpoints. An important function of such an agent is to advertise the real-time power capabilities and operational preferences of the supercapacitor array based on local information. Given the small energy capacity of such a device, its internal state can largely vary from one setpoint implementation to the next one. For this reason, the use of an accurate model is crucial in the agent definition. We show that it is possible to infer the real-time power capabilities of the device by using simple measurements on the supercapacitor array suitably coupled with an accurate representation of the cells composing the array. Results show that the agent is able to speak for the resource, thus allowing its use from an external controller