So the only complaint was $350 - $400 electric bills? No comfort complaints?
Heat pump or straight elect heat?
Utility bills averaged or was this summer only bills?
How many occupants are doing how many loads of laundry and opening & closing doors to let the house pets out to pee?
I don't think a all electric 2700sq.ft. house that has straight elect. heat with average billing is that far out of line by having a $350-$400 elect. bill.
The real problem here is the homeowner is trying to support too large of a house and ALL the bills are eating them alive. Kinda like your eyes are bigger than your stomach sorta thing.
Ok now let us assume that the bills could be lower if deficiencies are found. Maybe the indoor equipment is in the attic and this was not added in the misc. section of the load calc. Maybe the bottom of the door's weather stripping is torn off and the ceiling penetrations aren't sealed allowing a natural convection infliltration. No radiant heat shield in the building envelope would allow a hotter attic.
Heat strip stuck on during cooling mode?
Load calc calls for 37,000 btu so a three ton is selected but is only rated for 28,000 btu.
Load calc is done by a novice that thinks all there is to good design is plugging numbers in a computer program without using some general empirical common sense. In other words an inaccurate load calc was done.
Customer sets stat back during the day and it never catches up in the evening.