An energy-harvesting generator provides energy for storage in a capacitor. A sensing circuit senses a voltage across the capacitor and generates an activation signal as a function of the sensed voltage. The activation signal is switches from a first value to a second value when the sensed voltage reaches an upper threshold and switches from the second value to the first value when the sensed voltage reaches a lower threshold. A signal transmitter powered by stored energy in the capacitor responds to the activation signal being switched to the second value by activating and transmitting a transmission signal. The signal transmitter further responds to the activation signal being switched to the first value by discontinuing transmission of the transmission signal and deactivating. A duration of time elapsing between de-activation and activation of the transmitter is indicative of an amount of energy harvested by the energy-harvesting electric generator.