A heat load calculation adds up every source of heat entering the cold space. The main parts are transmission through the walls, doors and glass; infiltration of warm, humid air when doors open; the product load from warm goods placed inside; and internal loads such as fans, lights, anti-sweat heaters and defrost heaters.
Each part changes with use. A reach-in on a busy line opened every minute during lunch has a far higher infiltration load than the same unit in a back store. A glass-door merchandiser in direct sunlight near a shop entrance gets extra heat through the glass. A freezer loaded with deliveries that sat in a hot van carries a large product load until they are frozen again.
For plug-in cabinets, the manufacturer has already matched the system to a typical load at a stated ambient. For cold rooms and remote systems, a proper heat load calculation is the basis of correct sizing, and it should use realistic Indian summer conditions, delivery quantities and door use rather than averages.