I've been using a 9-volt battery as the power source, the battery lasts the whole growing season but the voltage drops to 5 volts or so by the end of the season (my measurements). It's a good idea to check the volage once in a while to make sure the battery is still providing enough voltage (corrosion can be a problem at the battery connectors). A few years ago when setting up a new fence I didn't have a 9-volt battery connector so I used a multi-battery AA holder that I had on hand. The combined series connection of batteries gave 12 volts. This turned out to be a bad idea as the 12 volts would more often than not kill the slugs rather than just turn them away.
The nice thing about the slug fence is that it is on duty 24-7, many other slug control methods require some kind of regular checking or resupplying. Also, no dangerous chemicals to worry about with this setup.
My first electric fence was mounted on a low "raised" bed garden. It was effective until the vegetation in the garden grew high enough to bend over and touch the ground - this gave the slugs the bridge they needed to reach the main feast.
The video below demonstrates how a slug typically reacts when attempting to cross the fence. It is interesting to notice from the video that once the slug was "shocked" a number of times (by making contact with both wires) it was then conditioned to react the same way when touching just one wire (no current flow possible through the slug).