 Energy conservation is becoming increasingly important in today's world, with governments and businesses alike looking for ways to reduce their energy consumption. One way to do so is through thermomodernization or deep renovation of existing buildings to make them more energy efficient. This process involves replacing old systems with new ones that use less energy, such as compressor heat pumps and photovoltaic panels. By doing so, energy savings of up to 80% have been achieved, resulting in significant cost savings over time. This case study examines one such project in Poland, where a school building underwent thermomodernization. The results showed that the building's energy efficiency improved dramatically, leading to substantial cost savings. Additionally, the project reduced carbon dioxide emissions by 74%. This article was authored by Anna Barwinska-Malajowicz, Radoslav Pyrek, Christoph Siksaka, and others.