A photovoltaic energy storage system, comprising one or more conversion circuits (11, 12, 13) and a detection circuit, wherein the one or more conversion circuits (11, 12, 13) are connected in parallel between a positive bus and a negative bus; the detection circuit comprises a signal source excitation branch, a first sampling circuit, and a control circuit; one terminal of the signal source excitation branch is connected to a ground wire, and the other terminal of the signal source excitation branch is connected to the positive bus or the negative bus; the signal source excitation branch comprises an alternating current signal source and a sampling resistor that are connected in series; and the first sampling circuit is connected in parallel to the sampling resistor, and the control circuit is connected to both an input terminal of the alternating current signal source and an output terminal of the first sampling circuit; the control circuit is configured to control the alternating current signal source to output harmonic signals of a first frequency and a second frequency, wherein the first frequency is different from the second frequency; the first sampling circuit is configured to: when the alternating current signal source outputs the harmonic signal of the first frequency, collect a voltage at both terminals of the sampling resistor to obtain a first voltage; when the alternating current signal source outputs the harmonic signal of the second frequency, collect a voltage at both terminals of the sampling resistor to obtain a second voltage; and send the first voltage and the second voltage to the control circuit; and the control circuit is further configured to obtain, based on the harmonic signals of the first frequency and the second frequency, the first voltage, and the second voltage, a ground insulation impedance obtained after the one or more conversion circuits (11, 12, 13) are connected in parallel. the detection circuit comprises a signal source excitation branch, a first sampling circuit, and a control circuit; one terminal of the signal source excitation branch is connected to a ground wire, and the other terminal of the signal source excitation branch is connected to the positive bus or the negative bus; the signal source excitation branch comprises an alternating current signal source and a sampling resistor that are connected in series; and the first sampling circuit is connected in parallel to the sampling resistor, and the control circuit is connected to both an input terminal of the alternating current signal source and an output terminal of the first sampling circuit; the control circuit is configured to control the alternating current signal source to output harmonic signals of a first frequency and a second frequency, wherein the first frequency is different from the second frequency; the first sampling circuit is configured to: when the alternating current signal source outputs the harmonic signal of the first frequency, collect a voltage at both terminals of the sampling resistor to obtain a first voltage; when the alternating current signal source outputs the harmonic signal of the second frequency, collect a voltage at both terminals of the sampling resistor to obtain a second voltage; and send the first voltage and the second voltage to the control circuit; and the control circuit is further configured to obtain, based on the harmonic signals of the first frequency and the second frequency, the first voltage, and the second voltage, a ground insulation impedance obtained after the one or more conversion circuits (11, 12, 13) are connected in parallel.