Web suite of building energy simulators that model energy, comfort, and daylighting performance, and give estimates of the long-term cost of utilities. The simulations restrict flexibility in order to offer users greater ease-of-use and speed. The tool can be quickly mastered by non-technical designers, and runs fast enough to allow them the scope to experiment with many different versions of a design during a single sitting. The immediate feedback that the site provides makes it useful in the conceptual phase of design, when architects cannot afford to invest large amounts of time to rule out any particular idea. The emphasis of the energy model is on the envelope system of the building, and includes simulations of high-technology windows such as double-skin facades. Energy-load estimates are based on a library of climate data for 30 different cities around the world.