In order to properly size and consider heating and cooling equipment options for large portable and habitable military vans, we developed a Fortran computer program that calculated the environmental heat gains (or losses) due to solar and ambient effects. Within the program these long rectangular cross-sectioned containers were subjected to conditions typical of a world-wide deployment, whose algorithms considered mean and extreme climatic conditions of wind speed and air temperature, as well as the specific solar effects due to external color, altitude, latitude, time of day, month, and year.

The actual heat load calculations were significantly simplified through the utilization of a U-factor (combined overall thermal conductivity of the external walls) and an ASHRAE-established solar heat transfer computation method method known as TETD (Total Equivalent Temperature Differential). As a result the radiation effects that acted upon all surfaces of the container were reduced to an equivalent external air temperature that through conduction would give the same net effect as radiation. Once metabolic, lighting, and fenestration effects were added to the calculated environmental loads the worst case conditions were bounded, allowing for sizing and qualification of the heating and cooling equipment.