Heat loss/heat gains calculations can be calculated using the methodology described within CIBSE Guide A, using a steady state model.
This type of calculation will output figures accounting only for heat losses gains through the building fabric and heat loss due to infiltration. In respect of heat gains calculations, solar gains are also accounted for.
The assessment is a steady state calculation which assesses the amount of supplementary energy required through either space heating or cooling to maintain a comfort temperature set-point.
It does not account for ventilation system heat gains/losses, dynamic behaviour such as variations in HVAC systems and room set-point temperatures, the effect of thermal mass, changing solar gains during the day, internal gains etc.
Clients receiving information in this format will be required to add or subtract system capacity to calculate appropriately sized HVAC systems.