An external power storage adapter (PSA) may supply electrical power to a portable information handling system or other power consuming device. The power storage adapter may include an AC-DC converter, a battery, and intelligent power management functionality. A PSA controller may determine whether electrical power supplied by the power storage adapter should be sourced from the AC-DC converter or from the battery based on various operating parameters of the power storage adapter, such the temperature of the power storage adapter, the state of charge of the battery, the amount of electrical power being supplied by the AC-DC converter, or the amount of time that the AC-DC converter has been supplying electrical power. The power storage adapter may output power to a port of the portable information handling system, such as a USB Type-C port, over a variable power bus.