Knowledge about energy requirements (ER) of dogs is important in order to provide dogs with an appropriate energy supply. In this study, the oral ¹³C-bicarbonate technique (o¹³CBT) was used to estimate energy expenditure (EE) in 50 privately owned dogs of different body sizes (small: Danish–Swedish Farmdog (DSF, n = 16), medium: Beagle (n = 15), large: Labrador Retriever (n = 19)), of different ages (adult: 2–7 years (n = 33), senior: ≥8 years (n = 17)). The aim was to evaluate the feasibility of the method in the field and to get more information about ER of dogs. All dogs were measured twice, on two separate days under standardized, resting conditions. The dogs were fasted overnight prior to measurements, which were carried out in home environment. Only measurements of dogs resting calmly were included in the statistical analyses. No significant difference (p > 0.05) in EE was found between days of measurements. The EE measured in Labrador Retrievers (405 kJ (97 kcal)/kg BW^0.75/day) was significantly lower (p < 0.001) than of Beagles (530 kJ (127 kcal)/kg BW^0.75/day) and DSF (497 kJ (119 kcal)/kg BW^0.75/day), the difference between Beagles and DSF being non-significant. Senior dogs had significantly lower (p < 0.001) EE values than adult dogs with least square means (LSM) of 441 kJ (105 kcal)/kg BW^0.75/day and 513 kJ (123 kcal)/kg BW^0.75/day respectively. Analysed within breeds, EE of senior Labrador Retrievers (370 kJ (88 kcal)/kg BW^0.75/day)) was significantly lower (p < 0.01) than adults (439 kJ (105 kcal)/kg BW^0.75/day), as well as in DSF (453 kJ (108 kcal)/kg BW^0.75/day and 541 kJ (129 kcal)/kg BW^0.75/day for senior and adult dogs respectively). However, the EE was not significantly different (p > 0.05) between adult (548 kJ (131 kcal)/kg BW^0.75/day) and senior (499 kJ (119 kcal)/kg BW^0.75/day) Beagles. This study suggests that when measured under standardized resting conditions, the o¹³CBT can provide reliable results of EE and be a helpful tool to get more knowledge about ER of dogs of different sizes, breeds and ages.