An apparatus using reconfigurable integrated sensor elements with an efficient energy harvesting capability is described. Each sensor element has sensing and energy harvesting mode. In the sensing mode, the sensor element measures an environmental characteristic by generating electrical charge and outputs a time-encoded signal indicative of the measurement. In the energy harvesting mode, the sensor element itself is used to harvest energy from ambient energy source and makes it available to other sensor elements or circuit components. The sensing element is switched from the sensing mode to the energy harvesting mode when the electrical charge reaches a predetermined threshold. An image sensor device using asynchronous readout for harvesting energy from incident light while generating images is also described.