I've encountered a couple of occasions where the mains power has been cut to the Kemper for around one-second and it has not affected the unit.
I'd assume that others out there have experience a similar situation.
Obviously part of the power supply depends on holding a charge as part of the circuit which accounts for this.
Now, i'm not suggesting that anyone experiments at the risk of damaging their unit but i'd be interested to know the longest power-cut that doesn't affect the Kemper.
My guess is that a powered-head will be shorter than an un-powered one, owing to the extra load, and maybe the rack unit will last longer than a toaster as there less LEDs to power.
There's probably other factors involved that i've not thought of too. I'm sure the tec-team have an accurate answer, knowing the component values of the power-supply circuit.
I'm led to believe that the Kemper can recover normally from a power loss (ie without the proper shutdown procedure)
disclaimer: Do not experiment at the risk of damaging your unit!