If you do a search on Back up power supplies, etc...been discussed many times. Yes, if you have a back up power on the stove, you Definately need something on the power vent to keep the fumes from backing up into the house. The stove should have a safety shutoff, if the power vent fails, it will stop the stove.
If you do not have backup power to the stove, the stove will go out pretty fast once no power is available to feed the fire. My direct vent Keystoker goes out within 10 mins or so after power goes out.
The stoves themselves are sealed pretty good, you may get some fumes out the baro (if you have one), but it shouldn't leak too much, since there is no flame with no combustion air no CO.