The period of time during which the output voltage will stay within its specified regulation in the event of the loss of the input power.

Determined by the storage factor within the power supply. In linear supplies, the time output fails when AC input fails, is almost immediate. In switching power supplies, energy is stored in the bulk (input electrolytic) capacitor providing a useable hold up time to protect against transient power outages. Since this is highly proportional to AC voltage and DC load, it is given under specified conditions. High AC voltage and low DC loads provide the best conditions.

The length of time a power supply can operate in regulation after failure of the AC input. Linears have very short hold-up times due to the CV squared energy storage product of their low voltage secondary side output capacitors. Switchers have longer times due to their higher voltage primary side energy storage capacitors.