I wouldn't consider running one along side air conditioning. The A/C does a fairly good job of drying out air. Also, if you look at it as the 500w space heater it is, it's not hard to see why your bills are higher. You're paying once to generate that heat, and (roughly) two more times to remove it from your home. Unless you can make good use of that heat, a more efficient approach might be moving that damp air to where your A/C can pick it up.