The Federal Park Service says it has to be 6 jouls
Interesting... I'll admit I haven't used a bear fence yet, but the kits places are selling don't have 6 joule fence chargers powering them. Many of the kits use the 4 D cell battery chargers which are well under a joule. If you look at the fence chargers in the kits, you can find the same units at farm stores.
For a short period you can power a 110VAC fence charger with an inverter, but you will likely need to protect the inverter from rain. I wouldn't run it off the car battery, but a house battery in the TD could be used. The inverter could then power small chargers, laptops, etc.. Using a 12VDC powered one would be better in the long run. My IntelliShock 52B will use a maximum of 5 Amp hours a day on a heavily weedy fence. On a weed free fence it will use less than 1 Amp hour a day. An AC fence charger powered through an inverter would likely draw allot more.
Just a bit of a info I figured out awhile back: Premier1 sells one of the fence charges I know is used by one of the kits. The case and model number has changed, but the stats are the same as the old SG50 unit seen in the UDAP kit. The Speedrite AN90
http://www.premier1supplies.com/detail.php?prod_id=51667&cat_id=43 is only 0.08 to 0.09 joules. It can be powered by 4 D cells or a 12V lead-acid battery. If you look at this page from UDAP
http://www.udap.com/bearshock.htm you will see the SG50 fence charger, minus the quick setup post. The Zareba 10 Mile Battery Operated Low Impedance Fence Charger
http://www.zarebasystems.com/store/electric-fence-chargers/b10li (also known as the "Yellow Jacket Fencer") is a similar unit, but over twice as powerful at 0.25 joules output. It is used by some bear fence kit suppliers.
I plan on using one of the IntelliShock 52B units
http://www.premier1supplies.com/detail.php?prod_id=373&cat_id=43 I use on the farm. They put out a wallop, but also cost allot, >$300. It would likely be massive overkill for a short bear fence as I only use 2 of them for my whole farm. I keep a spare one around on the farm incase a unit gets taken out by lightning. For power, I'll run it off the house battery in the TTT.
For fence posts:
http://www.premier1supplies.com/detail.php?prod_id=440&cat_id=47 I use the 35" gray ones on the farm for temporary fences. They are the longest lasting ones of all that I've tried, only three to four seasons, but that is two to three seasons better than any of the others. The secret is lots of UV inhibitors in the plastic. For a bear fence I'd get a set of the 48" ones. I'd space them every 4 to 8 feet. 4' when rounding a corner, and 8' along the sides. Buy a few extra as the clips eventually will break, but if you are careful they will last. Don't bang them hard. The fiberglass rod posts tend to splinter and you get fibers in your hands when handling them, even when new. The insulator clips for the metal posts die unless you use t-posts and use quality insulators. Then you need a fence post pounder and fence post jack as part of your kit.