We use Gallagher, procey, but well worth the money. The voltage/joules/acres/miles (everybody uses a different criteria) depends on the size of the area to be fenced. With a full load 5 to 6 k will work once their trained to it. Ours does 7.5k and with the drought and very poor soil conductivity for the ground we're still rading 5.7 to 5.9. The attached link has good information what ever brand you choose.
We use a 12v system called "battery saver" by Holmen. It has a high and low setting so that once your animals are trained you can use the low setting and the battery life is almost doubled. It also has a photo cell so that at night the charger pulses slower, also saving battery power. There is a switch to overide the photo cell function if needed. The cost is also very reasonable.