Hi, I hope you realise that there are some standards in place that define the performance of an electric fence unit.
They are defined to protect the body and the environment from possible fires.
The old method of using a car ignition coil is not the way to do it, there are energy and voltage limits to adhere too.
Please check in your country if there are standards as well, car ignition coil is a very bad idea and can be lethal.
Tom...... 
PS An electric fence spec as per standards.
(Standards for electric fences needed in Australia to help prevent death and bush fires.)
http://www.thunderbird.net.au/pdf/instructions/universal_energiser_instruction_booklet.pdf