Yes, if its HOB, pull the intake tube out to stop it from sucking water out of the filter and back into the tank, then put the tube back in, and fill the filter up with water. I learned that the hard way after dumping about 4 filters worth of water into the filter and scratching my head as to why it wasn't filling up.
Shouldn't be a problem. Last month my power was out for about 8 hours, my tank was fine. Temp only dropped about 3 degrees.