This paper presents data on the daily and seasonal changes in the inflow of solar radiation to the working surface of the photobioreactor. The results obtained correspond approximately to the reference data indicated in the literature for our geographical latitude for 1966 and 1988. Measurements were carried out from September 2019 to June 2020 inclusive on the basis of the Department of biotechnologies and phytoresources of the A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS, Sevastopol. A previously developed automatic light sensor based on an Arduino microcontroller was used as a measuring device. The main element of the sensor was a CdS photoresistor GL125. It is shown that the maximum absorption of the photoresistor is 593 nm, the sensitivity is 1.2 times higher than that of the luxmeter. To convert illumination units into irradiance, we used solar spectra at different heights above the horizon for the geographical latitude of Sevastopol. The average coefficient of conversion of klk in W/m2 for the used photoresistor was 4.26. The amounts of energy photometric quantity of irradiance in the blue, green and red ranges of the solar spectrum were determined. For our conditions it is shown, that in winter the number of red rays in sunlight is 1.22 times higher, while in the blue part there is a decrease in light energy by 1.5 times. The average calorific value of 21 kJ/g and the photobiosynthesis efficiency of 5.58% were used to evaluate the limit productivity of microalgae culture. During the observation period, the minimum value of limit productivity was noted for December - 133 g/m2, and the maximum for June - 824 g/m2. The results obtained can be considered as a certain limit in the organization of industrial production of microalgae in the southern regions of Russia.