BJCs require a lot of electricity, in a few areas.
You need to count up well in advance exactly what your power requirements are, because you will need to hire in the right generator, cables, distro board, etc.
The big top will generally require a 63amp supply.
The marquees will need multiple 13amp supplies at 240v for internal lighting, to power the fridges, pumps etc. for the bar, and any bands/sound system will also require power.
Gas space heaters may run on 110v, so transformers are required to drive those, plus each transformer needs a 13amp supply.
Festoon lighting for around the marquees and in the camp site may run on 110v so this also requires a transformer.
The Traders require power as well, many rely on laptops and phones to process credit card payments.
Portable toilets and showers
Portable loos / showers will require power of some kind, you will need the right cabling to connect it up. While some toilets don't need power, a shower will generally require a 63amp supply to run the heater for the water. Gas could be used to power the showers, here are some interesting calculations for that 
If you have an outdoor caterer, they will need power for their fridges.
Where to get it from
Large venues that are used to running events may be able to supply your power requirements. Smaller venues, schools, and so on are not likely to be able to provide 32/63 amp supply and may not want you using their power in any case. In this case you can hire in a (hopefully silenced) generator. Either way, you will have to plan your site layout so that all the things requiring power can be safely and cheaply reached from the power supply. You should also make sure you know how to restart your generator if it runs out of fuel in the middle of the night, and have the necessary keys for accessing control panels.
If you are hiring in a generator, be sure to budget for the fuel cost of running it constantly for almost 100 hours.
Setting it up
Water and electricity do not mix - cabling should be rated for outdoor use where necessary, and any distribution points (such as power strips) placed where they cannot get wet.
Electrical sockets should be kept out of the reach of the convention attendees where possible, otherwise people may attempt to unplug things at random to charge mobile phones, PDAs, MP3 players and laptops. You may want to consider providing a dedicated area for these - indoors rather than outdoors. Do not daisy chain power strips together as this may result in overloading power sockets.
Individual items (kettles, stereos) should be PAT tested. Have a supply of plug fuses somewhere handy for those vital items.
Find out where the main breakers are and what to do in the event of them tripping - if the site plunges into darkness you may need to get it switched back on in a hurry. If you aren't allowed to do this yourself you will need an emergency number for the site's caretaker or whoever has access. Remember that breakers may have tripped for a good reason, so try and find out why if you can.
In order to keep everything safe there are some nice standards for electrical connections. It is almost impossible for example to connect a 110v plug to a 240v socket. The different voltages and amperages are all colour coded, have mis-matched keyways, and are different in size. However never assume wiring has been done to the standard and double check that you're connecting what you think you are.
Joule's law can help you work out your requirements if you're not sure.
A 240v 13amp supply will give no more than 240*13 watts = 3120W or 3.120kW, so if nothing else is being drawn from that supply you could power a 3kW sound system from one 13amp supply, or two 1kW systems and 1kW of lighting (or one kettle!). If in doubt - ask someone who knows.
As with anything where error can result in DEATH - if in doubt seek help.