Why is the SACC BTU rating lower than the advertised BTU?

SACC (Seasonally Adjusted Cooling Capacity) is the DOE's standardized measurement and is what actually predicts real-world cooling. The bigger 'ASHRAE BTU' number on the box is closer to lab-ideal conditions. When comparing units, the SACC figure is the one to trust — a '14,000 BTU' unit is often 9,500–12,000 BTU SACC.