Is it worth to insulate our building thickly? If we put a solar panel on it, could it replace the heating? Will it really be possible to heat with a heat pump? Can we use our electric car as an energy storage device? Dynamic building energy simulation gives a detailed and clear answer to these and similar questions. Dynamic here means that hourly meteorological data, terrain, and the built and natural environment for a particular building become part of the model calculation. This calculation provides an answer to any usage situation that may occur with the building. It even provides an opportunity - while meaning significant savings for investors and owners - to compare different scenarios and technical solutions without having to build them. It responds to the highest levels of comfort, including visualization of changes in natural light, as well as thermal comfort, which is what we can expect from a modern building today. It makes you decide which mechanical design represents the best economic decision, it responds to expected operating costs, we will see how many hours a year the ventilation equipment operates and the cost of heating can be accurately estimated. It Makes the effects of changes during construction predictable. In everyday practice, the AA or AA + building energy rating is best achieved through a dynamic building simulation model. Summarized in one sentence, dynamic building simulation makes the cost implications of our decisions, the future cost of operating buildings and the expected comfort services visible and quantifiable.