A Manual J load calculation is the industry-standard method for determining exactly how much heating and cooling a specific home needs. Published by the Air Conditioning Contractors of America (ACCA) as ANSI/ACCA Manual J, it accounts for the home's square footage, insulation, windows, orientation, air leakage, and local design temperatures to produce a room-by-room and whole-house load — the basis for selecting equipment of the right capacity. National building codes and most jurisdictions require it for new HVAC work.
Correct sizing matters more for a heat pump than for almost any other system. Oversize it — the old 'round up a ton to be safe' habit — and the unit short-cycles in mild weather, which the ACCA notes leaves indoor air 'cold and clammy' because it never runs long enough to pull humidity out, while the constant stop-start wears the equipment and lowers efficiency. Undersize it and the home leans on expensive backup heat through much of the winter. The whole point of modern variable-capacity heat pumps — matching output to the actual load — is lost if the load was never calculated.
Manual J is the first half of the job; Manual S (equipment selection) and Manual D (duct design) follow from it. For a buyer, a Manual J calculation is the single best signal that an installer is engineering the system rather than eyeballing it. Ask to see the load calculation, not just a tonnage; a quote that jumps straight to 'you need a 4-ton unit' without one is a quote based on habit, and an over- or under-sized heat pump underdelivers on both comfort and the efficiency you paid for.