Power is provided from a power source and a status of the power source is signaled by controlling a waveform of an AC voltage generated from the power source. The status may include, for example, a capacity of the power source. In some embodiments, a frequency of the AC voltage may be controlled to signal the status. The power source may include, for example, an uninterruptible power supply (UPS), and signaling a status of the power source may include controlling an inverter of the UPS to signal the status. Related systems are also described.