The MTWAS embodied energy calculator is based on the "Concept Model" presented in the Advisory Council on Historic Preservation report, ASSESSING the ENERGY CONSERVATION BENEFITS of HISTORIC PRESERVATION: Methods and Examples. This report presents three techniques to calculate embodied energy. Each model measures the energy embodied in the existing building's materials. The concept model approach is simple, with only a single calculation required: invested energy per sq. ft. of the building type, multiplied by the gross floor area of the historic building. The values in MBTU/sq. ft. for each building type are presented to the right, as published in the ACHP report.