Methods and systems are provided for diagnosing the electrical efficiency of an electric supercharger staged upstream of a turbocharger. During idling conditions, a step-wise incremented signal may be commanded to an electric motor driving the supercharger compressor and changes in compressor speed and total vehicle current may be measured at each step. Degradation of electrical efficiency of the supercharger may be inferred based on deviations between the actual change in compressor speed and total vehicle current relative to the change expected for the given step, and mitigating actions may be accordingly performed.