A backup computer power system for powering and controlling a computer which is otherwise powered by a main power source, when the main power source fails or is disconnected, wherein an auxiliary power supply provides power to the computer and an instruction set in the computer controllably shuts down the computer after first allowing optional operator intervention prior to initiation of the programmed shutdown. If the operator chooses to intervene, he can permit completion of the current operations or conduct a manual shutdown procedure. If the operator does not intervene within a predetermined period of time, the instruction set terminates the current activity of the computer, saves the software operating environment of the computer, and shuts down the computer. Optionally, the instruction set can then shut off the auxiliary power supply to save its capacity for later use. When the main power source is restored, the auxiliary power supply signals the computer, and the instruction set initiates normal startup within the constraints of the saved operating environment.