A method of controlling a power system provides at least two alternative sources of power delivering power to a power bus. Switches are provided between each of the at least two sources of power and the power bus. The bus is powered by a first of the at least two sources of power. A characteristic of the power supplied by the sources of power is monitored. Switching from the first source of power to a second source of power occurs by opening a switch associated with the first source of power, and closing the switch associated with the second source of power. The timing for performing the switching is selected such that one or more characteristics of the power supplied by the first and second sources of power are close to each other to minimize the apparent break in the power. A power system incorporate controls for effecting the method.