Voltage sags and interruptions, which can cause loss of stability of the load node, are the most dangerous power failures for oilfield power consumers. The purpose of the research was to assess the sustainability of the oilfield power consumers by developing a safety factor taking into account the dependence coefficient of power sources. In this paper the safety factor has been proposed on the basis of the area method. The influence of the power supply dependence coefficient and the setpoint for automatic reserve entry time on the safety factor has been analyzed.