Waste incineration is one of the paths of implementation of the European Union’s strategy aimed at reducing the amount of waste deposited in landfills. Along with the development of methods for processing and recycling various wastes, new waste is generated. One example is waste from polyethylene terephthalate (PET) bottles washed during their recycling. In this paper, physical and chemical properties of such wastes are analyzed in terms of their use in the power industry. This research is part of the search for new sources and new technologies for energy production. The study has taken into account the energy properties of waste intended for combustion (calorific value, water content, chemical composition, volatile substances, combustible and non-flammable content). Thermogravimetric analysis of the material tested indicated that the waste is a good source of energy. It was found that the elemental composition (C, H, N, S, O) of the waste investigated is similar to that for biomass materials, and the calorific value of 13.2 MJ/kg qualifies the waste for combustion, provided that its initial moisture is reduced, for example, for co-combustion in the cement industry. Another possibility is mixing the waste with other kinds of waste to obtain a new fuel with more satisfactory parameters.