My orginal safe came wired with outlets so I've used golden rod units with it. My second, larger safe, wasn't wired. So I used one of the mounting holes in the base and ran a line thru it and sealed with caulking. I am not really thrilled with my new safe, it doesn't seem to have as good a door seal on it. So being the paranoid that I am, I also have one of the Remington units I found on sale in it.
It seems to work well, but as Magic said, they seem to need recharging pretty frequently. And lately I have been giving some thought to a room dehumidifier. My safes are in an area with a concrete floor and we have a humidity issue because of a running spring under the house. I have a haz-mat cabinet I found at a great price for ammo storage, so I really think the room dehumidifier is in my future. The local Essex store just got in some LG and Sears units selling for about 30 bucks, so that's my plan. If anyone is interested, I'll post my thougts on the results later.