A Manual J load calculation works out how much heating and cooling your building actually loses and gains, room by room, using its real dimensions, insulation, window area and orientation.
Why oversizing hurts
An oversized system satisfies the thermostat quickly, then shuts down. Short cycles wear the compressor, never run long enough to pull humidity out of the air, and leave you with a house that is cold and clammy at the same time. It also costs more to buy.
Any quote that arrives without a load calculation is a guess dressed up as a specification. Ask for the sheet.
