Hi,
I have a battery powered energiser that will do 5km of wire,it's fully weather-proof and hangs on the post by the gate,(easy to switch off in an emergency. It cost £95 and included the earthing spike and 400m of white nylon/metal rope. I run it off a car battery of 65amps,and have a spare inside the shed on trickle charge. One battery lasts about 3 weeks on permanently if there's no shorts with vegetation. I also have a solar battery charger (£10 from Maplins) and that extends the 3 week period if it's sunny. The running costs are minimal.
I did look at mains energisers,the area I wanted to fence in is 30metres from the house,but the cost of the armoured cable to run that 30m,digging a trench a foot deep and all the sand ontop of the cable in the trench etc,all added up to a lot of expense and effort,plus the mains energisers are generally more expensive. Obviously,your personal situation might dictate which you use,either way,build in a switch or breaker in an easily accesible place..... Ree