The building sector constitutes a significant share of global energy consumption and CO2 emissions. Despite improving technology and materials, the foundation of successful energy design lies in architectural design decisions. Windows are one architectural design issue that is a key factor in both energy efficiency and functionality. This research utilized dynamic building simulations to study the direct and combined energy impacts of window related architectural design through the following main variables’ effect on heating, cooling, and lighting need: window area, proportions, horizontal position, external shading, glazing properties, and adjacent room proportions. Four solar orientations and three climatically different Finnish locations were included as context variables. Window energy efficiency in the studied cases was found to be primarily about controlling solar heat gain. Geographic location changed not only energy consumption directly, but also the individual variables’ impact on it and each other. The results indicate that cooling can be a significant factor even in cold climates, and that energy efficiency should always be evaluated using local weather data as well as sufficient design detail. Most properties affected each other significantly, highlighting a need for a comprehensive approach in both research and design practice and allowing evaluating previous studies comprising fewer variables.