Hi Lee,
Elddis (formerly Explorer Group) recently produced an all-electric caravan and, I believe, sales were disappointing.
There's no real limit to the amount of electric you consume on site - though you won't be able to draw more than 16 amps (in fact on a lot of sites you may be able to draw as little as 4 amps).
However, you really shouldn't be looking at drawing as much as you possibly can - the electric hook-up charge is based on people drawing a "reasonable" amount.
If people turn up to use as much as they can then sites will be forced to increase their prices, or remove their electric facility.
Due to the increasing levels of electrical equipment fitted to caravans, some sites have even started metering their supply.
Remember, all the electricity you use the site-owner has to pay for. So, while that may not trouble your conscience on a large national chain of sites, it will be a different kettle of fish for the owner of a small CL who may be charging you under a tenner a night for the privelege.
In other words - be sensible! Don't be selfish and don't take the p**s!!