While I wasn't able to find out a simple calculator of energy, I suspect that by using a catalyst, the energy should be manageable. Also I think the process would be scaleable, so in the beginning they would only try to make a little ammonia and other molecules to use in controlled enclosed areas and they would use the spare energy from the Nuclear Generators (running at the same peak rate) which would go to waste in the days when Solar Panels or Wind generators have higher yields. So basically ammonia production would be of secondary importance in the beginning. Plus, the energy technology will evolve, by the time we will need to do it on a full scale we will be farther than what we can imagine now.
As for shipping ammonia to Mars (if it can be safely shipped - I wouldn't want to be near the Landing Zone of the Ammonia Lander :D ) I think that it will be facilitated by reduced shipment costs due to reusable craft and other commercial interests in the area.
Very good questions, actually. Thanks!
Offhand, it's something like 1% of the world annual energy budget to meet the global demand for ammonia. There are catalysts which can reduce that, but they all have a slew of complications. If you can think of one which works well, do tell. We'll patent it and buy all the Steem.