Having to change our electric fencing daily at the moment to section off standing water/mud in their paddock and slowly let them into the good stuff. All a nightmare to be honest.
Anyway Saggy has explained it well. Speak to your stockists - we got our from CWG in Bury - and they will help you.
We've just updated our battery and have gone for a special deal where we got 2 smaller batteries. Works well because we've always got one on charge so can swap them without a changeover period.
Don't forget that as well as posts (large white ones give height options - we likes ours low for the ponies) you need to get an earthing post, the power alternator thingy which goes between battery and fence, black rubber rings for screwing into fences/trees and (easy to forget) a special battery charger unit for your house/stable. Also consider a special gate section if that makes life easier for you. In our Winter field we have a gate section from a post to the fence but in our Summer field we just tie a loop on the end of the tape.
We use wide strips of tape - mostly because they don't seem to get so messed up when you're rolling up or out plus more easily seen.
You need to think if you're going to have one or two rows of tape - we only use one but that is because our ponies aren't natural escapers.
Try to make the fence line as short as possible because the less tape you use the stronger the electric flow.
When you unclip the battery to let them out/change posts do the red gator clip first.