Although designers can get an idea of the future energy bills of a new house by checking those of comparable homes in the area, energy-modeling software is now used for most energy estimates. Some programs are free, others cost hundreds or even thousands of dollars, but all have their strengths and weaknesses. In this article, senior editor Martin Holladay looks at energy-modeling software in general, then provides information on a number of the most commonly used programs. He identifies two problems with using the software: First, it takes a long time to enter all the required inputs, some of which are difficult to assess and can seem unrelated to energy use. Second, because many relevant factors can't be modeled, the results of some programs may not be accurate enough to be useful. Moreover, energy use is driven not only by envelope design, but by occupant behavior. Still, most energy models are fairly accurate when applied to new homes. They are less so with existing homes. For builders unfamiliar with energy-modeling software, Holladay recommends trying a free program first, then purchasing a more robust program to better integrate energy modeling into the building routine.