First, it will depend on the species. Ant sizes cover such a huge range that the mass of the largest is thousands of times greater than the smallest and energy usage will depend on the size.

This study:

http://www3.interscience.wiley.com/journal/119878132/abstract 

provides data for one species which suggests a figure in the one kCal per day, though I suspect this number could be off by a large factor either way. There is also work that shows the per ant energy usage rate depends on the colony size since the heat produced and retained in the colony has an effect