This paper deals with an energy-autonomous wireless sensing system with an indirect measurement of the light energy for ambient lighting sensing. The system can perform power measurements without battery or external power sources and generates time-domain data. The advantage of the proposed solution consists of greater energy efficiency, a reduced number of components, higher miniaturization, and a reduction in implementation costs. The device is implemented using off-the-shelf components on a printed circuit board (PCB) with the size of 2 x 2 cm(2) and the thickness of 0.45 cm, it can harvest and detect sunlight ambient light as low as 200 lux.