A power module for a computer system includes an internal source of stored energy and a group of interface ports, including a management port and non-management ports. The management port provides a control interface to control the power module. The non-management ports each lack the control interface of the management port, and each provide a signal indicating that the power module has a sufficient amount of the stored energy to power a defined operation by external devices coupled to and drawing power from said non-management ports. The management port will typically provide a similar signal to the coupled management device. The signal may be implemented as a high or low voltage level on a serial interface cable pin.