Any condition in which the electricity supplied by a private or commercial power source is interrupted. For a UPS, a power failure is any condition that results in a loss of input power. Some variations on the concept of a power failure include: Blackout - A complete loss of power to the UPS. Brownout - A condition where the power voltage drops below the lower limit of the UPS, causing the UPS to switch to battery power via the inverter. Surge - A condition in which power voltage exceeds the upper limit of the UPS, causing a switch to battery power.