What size heat pump is needed for a 1600 square foot duplex?

Contrary to popular belief, the required BTU output, or "tonnage", cannot be calculated based on square footage. Many factors go into determining the required BTU output to compensate for your home's heat loss/gain. A properly trained HVAC technician has the ability to calculate the required heat load for your home based on where you live, the materials your home is built with, the "r" value of your insulation, window type/size/quality/quantity, and even which direction each exterior wall faces. Many "old school" technicians say 500sqft per ton, but with today's strict building codes and better insulation, this rule of thumb no longer applies. In most cases, the old rule will cause you to oversize your system, resulting in big problems down the road, i.e., high humidity, mold, mildew, etc.