CHVAC software calculates cooling loads using either the CLTD (Cooling Load Temperature Difference) method or the RTS (Radiant Time Series) method. The CLTD method is a traditional approach that estimates cooling loads based on temperature differences between indoor and outdoor air, while the RTS method is a more advanced approach that takes into account radiant heat gains and time-series data. The RTS method is considered more accurate, especially for buildings with complex geometries or high internal heat gains.