Agricultural biogas plant operations are energy sources that fit well with rural land use. The continuous increase in cattle and pigs creates the need to manage animal waste, especially slurry. Using it for energy production in biogas plants offers the possibility of obtaining electricity, heat, and highly efficient manure. The thermal energy generated in an agricultural biogas plant is used to heat the substrate in the fermentation process and can also be used to heat buildings, dry fodder, or wood. Electricity can be used to cover the farm’s needs and sold to an energy company. However, the energy generated in the biogas plant must be of the right quality. One of the main factors describing the quality of electricity is the voltage distortion from the sinusoidal waveform. This paper presents the results of a study of the impact of biogas plant operation on the course of voltage and current in the grid. The theoretical analysis of the voltage distortion mechanism at the point of connection of an agricultural biogas plant was based on a simplified power system model consisting of a voltage source and the equivalent impedance of the power system. According to the theoretical analyses, agricultural biogas plant operation should reduce the voltage distortion factor. In order to confirm this statement, field tests were carried out at three agricultural biogas plants, based on which an analysis was made of the impact of the power generated in the biogas plants on the value of voltage distortion occurring at the point of their connection. However, the field tests did not confirm the conclusion of the theoretical analysis. Only in one case (where the biogas plant was connected near a substation and there was the highest short-circuit power) could it be seen that an increase in generation affected the voltage distortion factor. In the other two cases, generation did not significantly affect the shape of the voltage waveform. However, in each of the biogas plants studied, as the generation power increased, the current distortion factor decreased, suggesting that agricultural biogas plants should operate as close to the rated power as possible.