A portable power supply (200, 300) configured to be operated on AC mains having a mains input (201, 301) comprises: an AC-to-DC converter (202, 302) coupled between the mains input and a rechargeable battery unit (205, 305), the AC-DC converter configured to convert the AC mains to a first-voltage DC power to be stored in the rechargeable battery unit; a DC-to-DC converter (204, 304), wherein the rechargeable battery unit is configured to be coupled to an input of the DC-to-DC converter and the DC-to-DC converter is bi-directional; a DC port (213, 313) coupled to the DC-to-DC converter for operatively connecting with a battery pack (206, 306) external to the portable power supply (200, 300); a power inverter (203, 303); and a housing (207, 307) for housing the AC-to-DC converter, the rechargeable battery unit, the DC-to-DC converter and the DC port; wherein the output terminal of the rechargeable battery unit is configured to connect with the power inverter having an AC outlet for supplying an AC output to an AC load; and wherein the portable power supply is configured to be operable and switchable between a normal DC operation mode in which the DC-to-DC converter converts the first-voltage DC power to a second-voltage DC power for charging the battery pack, and a discharging mode in which the second-voltage DC power stored in the battery pack via the DC-to-DC converter and the first-voltage DC power stored in the rechargeable battery unit are released to an AC load via the power inverter; the portable power supply configured to automatically switch between the discharging mode and the normal DC operation mode when the AC load is connected and disconnected to the portable power supply via the power inverter (203, 303) respectively. an AC-to-DC converter (202, 302) coupled between the mains input and a rechargeable battery unit (205, 305), the AC-DC converter configured to convert the AC mains to a first-voltage DC power to be stored in the rechargeable battery unit; a DC-to-DC converter (204, 304), wherein the rechargeable battery unit is configured to be coupled to an input of the DC-to-DC converter and the DC-to-DC converter is bi-directional; a DC port (213, 313) coupled to the DC-to-DC converter for operatively connecting with a battery pack (206, 306) external to the portable power supply (200, 300); a power inverter (203, 303); and a housing (207, 307) for housing the AC-to-DC converter, the rechargeable battery unit, the DC-to-DC converter and the DC port; wherein the output terminal of the rechargeable battery unit is configured to connect with the power inverter having an AC outlet for supplying an AC output to an AC load; and wherein the portable power supply is configured to be operable and switchable between a normal DC operation mode in which the DC-to-DC converter converts the first-voltage DC power to a second-voltage DC power for charging the battery pack, and a discharging mode in which the second-voltage DC power stored in the battery pack via the DC-to-DC converter and the first-voltage DC power stored in the rechargeable battery unit are released to an AC load via the power inverter; the portable power supply configured to automatically switch between the discharging mode and the normal DC operation mode when the AC load is connected and disconnected to the portable power supply via the power inverter (203, 303) respectively.