This article presents the energy balance conditions of three multi-family residential buildings located in Kryvyi Rih, Ukraine. These buildings were built more than 40 years ago and are not insulated. A significant amount of energy is lost by the external partitions. The building’s heating is powered by municipal natural gas. A proposal for several thermomodernization variants for selected facilities are presented in the article. These variants of deep thermomodernization include an upgrade of the heating installation, insulation of external walls and roof, and the replacement of windows and doors in common areas. In view of the recent increases in the gas prices, the internal building's temperature is maintained below the thermal comfort. The presented variants include the proposal to leave the internal temperature at the present level, as well as to increase it to 20 °C. Heat transfer coefficients were calculated based on of the PN-EN 6946 standard and the heat demand for the buildings was estimated in the standard reference year in accordance with the PN-EN ISO 13790 standard. Results are compared with real values. The buildings’ models were prepared in the Audytor OZC 6.7. Pro software. The impact of appropriately sized photovoltaic system for each of the objects was also analyzed and the solar irradiation data has been taken from the RETScreen 4 software.