Relevance. Active development of technologies for creating high-capacity electric energy storage devices facilitates their use in electrical complexes of oil and gas producing enterprises. The introduction of electric energy storage devices in such complexes allows increasing the reliability of power supply to consumers, improving the quality indicators of electric power, and also effectively using low-power power sources based on renewable energy sources. Aim. Development of methodology for calculating the parameters of an electric energy storage device of an electrical complex at oil and gas producing enterprise. Methods. Mathematical modeling. Results and conclusions. Based on the research results, the authors have proposed the method consisting of two independent algorithms for calculating the parameters. The algorithms differ in the sequence of calculations: the load reservation time and its value or the ratio of the load reservation time to the storage device charging time and the load value are used as initial data. The algorithms include an operation for optimizing a set of galvanic cells. The algorithms also contain an operation for selecting the type of galvanic cells, implemented in the form of calculating the efficiency indicator. The method was tested on the example of selecting the parameters of an electric energy storage device for installing an electric centrifugal pump of an oil and gas producing enterprise. For a load capacity of 123 kVA and a backup time of 15 minutes, the capacity of a set of galvanic cells with a nominal voltage of 12 V must be at least 3571 A∙h. According to the results of the operation for selecting the type of galvanic cells for 900 charge-discharge cycles of the storage device located in a heated room, it is rational to use lead-acid storage batteries. According to the results of the storage device optimization relative to the capacity value and the number of individual galvanic cells, their weight and cost, it is rational to use the following sets for the pumping unit: 24 galvanic cells with a capacity of 150 A∙h manufactured by Delta Battery, 55 galvanic cells with a capacity of 65 A∙h manufactured by WBR. For a full charge time of the storage device of 12 hours, the power of the power source varies slightly depending on the capacity of the set and is of 2607–2654 W.