I'm assuming you have an electrical meter? With as much as you can turn off, count the revolutions of the meter while they're on, individually, if you can. The scale will be printed on the meter. Now, if you have a newer, hi-tech meter, you may not be able to do that.
But, the data plate (likely underneath the access cover) should show what they draw. You'd need to add in whatever the air handler used as well, and if it needed auxilliary heat, those resistance heaters can really add up. One thing to keep in mind, if you try to do a significant setback overnight, because of the amount of heat needed to bring the house back up to temp (verses what it takes to just keep it there), you may find that it costs more to set back than it would to run the thing at a more constant temperature...if aux heat is triggered, the heat is a 1:1, not some multiplier of the heat pump that could be 3:1 or so. The duration and size of the setback would determine whether it's cheaper to just go stable, or keep setting back.