Solar energy radiation is essential in plant physiology and pathophysiology hence its knowledge is fundamental for example, to estimate evapotranspiration (Gocić et al., 2015; Petković et al., 2015) and to predict infection risk of some fungus diseases (Katsantonis et al., 2017; Dalla Marta et al., 2008) that are needed to schedule irrigation and fungicide spraying. Solar energy radiation is usually expressed in terms of the energy flux density through a horizontal area (irradiance) and an integrated value over one day (daily solar insolation) fits these applications. For precision agriculture, high accuracy measurements are not required and manufactures recommend the use of photodiode-based pyranometers which are cheaper than thermopile-based pyranometers (Kipp&Zonen, 2018). However, they still cost hundreds of euros and consume some milliwatts hence do not suit low-cost wireless sensor nodes. As an alternative, insolation values in field studies are usually obtained from public weather stations often far away from the crop of interest. This results in errors due to the inhomogeneous solar energy distribution caused by orography, competing vegetation or clouds (Reuter et al., 2005). For extended areas, more reliable in-field data would be better obtained from wireless sensor networks that include solar radiation sensors but this can be thwarted by cost and power consumption constraints (Wang et al., 2006). In order to overcome these constraints, we propose to use the components already integrated into the solar energy harvester of sensor nodes to measure solar radiation too. During the last decade, small PV panels have been used as low-cost radiation sensors to monitor PV solar plants. Solar irradiance has been deduced from the voltage drop across a resistor biased by a PV panel operating near short-circuit condition (Husain et al., 2011). Short-circuit current is approximately proportional to solar irradiance hence a way to estimate it, but unfortunately the power yield is null at this operating point. Further, the temperature drift of the sensitivity of the PV panel to solar irradiance must be considered. An obvious solution to compensate for temperature drift is to include a temperature sensor (Carrasco et al., 2014; Mancilla-David et al., 2014; Ma et al., 2017) but the sensor and its conditioning circuits add cost. An alternative solution is to measure the open-circuit voltage and the short-circuit current of the PV panel (Ortiz Rivera and Peng, 2006; da Costa et al., 2014). The temperature coefficient of the short-circuit current is positive whereas that of the open-circuit voltage is negative, and both increase with solar irradiance, which leads to a bijective function between them that can be obtained from a physical model for the PV panels. Unfortunately, the calculation is performed by iterative complex algorithms that require DSPs, and current sensors that do not suit low-power solar energy harvesters because of cost and power consumption constraints.