Even though your outlets have a plate cover, air can still pass through the outside walls, and sneak through the electrical box to the interior of the house. This can allow the cold or heated air to do the opposite of its purpose, causing your air conditioner to work harder.  To prevent this take a tube of caulking, preferably painters or paintable silicon, and seal the trim of all the electrical plates. This will prevent air infiltration.

Just remember all the small cracks combined in an entire home, can sometimes be equilavent to a golf ball size whole, perhaps a softball.