Renal energy losses of horses are high in comparison with other species. In the present study, more data were obtained on this parameter to improve predictive equations for renal energy losses. Four adult ponies (247–344 kg body weight [BW]) were fed with eight different diets based on first cut hay, second cut hay, early first cut fresh grass, late cut herbs–grass mix, early cut clover–grass mix, sugar beet pulp, rice bran and straw. Feed intake was measured, and urine and faeces were quantitatively collected for 3 × 12 hr at daytime and afterwards 3 × 12 hr at nighttime. Feed was analysed for crude nutrients, gross energy, amino acids and neutral‐detergent‐insoluble crude protein (NDICP); faeces were analysed for crude nutrients and gross energy; and urine was analysed for nitrogen (N) and gross energy. Renal energy losses per gram dry matter (DM) intake (y; kJ/g DM) were strictly correlated to protein content in DM (x; g/kg DM): y = 0.325 + 0.00431x; r2 = .81; n = 38; p < .001. The data suggest that the intercept represents energy losses by detoxification products such as hippuric acid and the regression coefficient by protein metabolites such as urea.