Aids in the design of single- or multizone energy-efficient buildings where the loads are dominated by the dynamic interactions between the building's envelope, its environment, and its occupants. SUNREL is especially well suited for passive solar buildings and includes algorithms for Trombe walls, advanced glazings, schedulable window shading, active-charge/passive-discharge thermal storage, and natural ventilation. The program is a true simulation model based on time steps of one hour or less. The model representation of the building is a thermal network solved with forward finite differencing among other techniques. In addition, a simple graphical interface aids in creating input and viewing output. Itï¿½s not very pretty, but it is easy to learn and review precisely what inputs went into each simulation run. SUNREL is an upgrade of SERI-RES, which was released in the early 1980s by the Solar Energy Research Institute (now known as NREL). The program has been used by researchers around the world and is the underlying building physics and mathematics package in TREAT, a program offered to all New York State energy auditors by the New York State Energy Research and Development Authority (NYSERDA).