What factor contributes to the determination of external heat gain?

The determination of external heat gain in a building primarily hinges on how much solar radiation impacts the structure. Given the context, solar gain refers specifically to the absorption of solar energy through windows, walls, and roofs, which subsequently influences indoor temperatures. The more direct sunlight that penetrates a building, the greater the heat gain, which is essential for calculating HVAC loads. Therefore, understanding the components of solar gain is vital for ensuring energy efficiency and comfort.

Other elements like internal appliance usage, conduction and infiltration gain, or cooling system efficiency play roles in overall thermal performance, but they do not directly address how external heat input is quantified in relation to solar exposure. Each of those factors contributes to the internal heat dynamics but does not focus solely on the external heat input stemming from sunlight.