Unit fuel is spent against the heat that enters the body and has to be removed, not against distance covered, which is why two vehicles driving the same kilometres can burn very different amounts. The ASHRAE method groups the refrigeration load into five parts.
| Heat load | What raises it on a route |
|---|---|
| Transmission (walls, floor, roof) | Outside temperature difference and driving time |
| Air entering at door opening | Number of stops and door-open time |
| Product | Cooling of product loaded above target |
| Pre-cooling | Cooling the body and inside air at each loading |
| Other | Internal sources and equipment |
In the city the door opens at every stop, the body air is replaced and the unit works at full power to recover; even over few kilometres, unit hours and fuel stay high. On a long haul the door stays shut and the unit mostly offsets heat crossing the walls. Waiting is a separate item: a unit with its own diesel engine burns fuel while the vehicle stands, and that consumption never shows in a per-kilometre figure.
