Composting is one of the most common methods of treating biodegradable waste, and application of the process’ final product into the soil is, due to the high carbon content, one of the most rational ways of its management. If the compost is not in line with legal requirements, it is necessary to search for alternative ways of its use, such as application for energy purposes. The aim of this study was to estimate differences in the physical, including energetic, properties of composts prepared from plant feedstock with additions of such materials as biochar, sewage sludge, coffee grounds and yeast effluent. The composting process was carried out for 140 days. The basic chemical and physical properties as well as heat of combustion were determined in the analysed feedstocks and mixtures derived from them. It was found that 10% addition of biochar had no significant effect on the composting process rate; however, biochar-amended treatments showed a smaller loss of dry matter and higher C:N ratio compared to other combinations. The use of biochar or coffee grounds as additives in the composting process reduced volumetric density of the composted biomass. Maize straw amended with sewage sludge and coffee grounds reduced air-filled porosity of composts. The share of biochar in the compost limited this tendency. The heat of combustion determined in composts was lower than the parameter determined in material mixtures before the composting process. The results show that sewage sludge reduced the heat of combustion of composts, which was closely related to ash content.