With constantly progressing climate change and global warming, we face the challenge to reduce our energy consumption and CO2 emission. In Europe, more than 1/3 of our total energy consumption results from buildings, where more than 30% of energy loss is due to dated inefficient windows. Here we present our development of the SunSmart thermochromic smart window for optimized solar heat management in continental and oceanic climates. Our proprietary smart window combines high visible transmission of up to 75% with high solar modulation of up to 23%. The material changes its solar heat transmission at a temperature of 20°C and transitions from an infrared transparent to blocking state, whilst staying transparent in the visible. In an optimized IGU this leads to a change in G value of up to 13% between the cold and the hot state. This translates to annual energy and cost savings of approximately 1000 kWh and 25 €/m2 glass (500 € per year) for an average household in The Netherlands. The thermochromic technology is complementary to current low-e coatings and can be implemented in regular windows and frames without additional requirements for special installations like wiring. In addition to optical performance and impact on energy, cost and CO2 emission savings, we present the first 1 x 1 m2 demonstrator installed in a test building with monitoring of the thermochromic performance in real environment. Furthermore, we present the next steps in scale up towards pilot scale production.