Biomechanical energy harvesters generate electricity, from human movement, to power portable electronics. We developed an energy harvesting module to be used in conjunction with a load carriage device that allows carried mass in a backpack to oscillate in the medial-lateral direction. The energy harvesting module was designed to tune medial-lateral oscillations of the carried mass to create favourable device-user interaction. We tested seven energy harvesting conditions and compared them to walking with the device when the weight was rigidly fixed to the backpack frame. For each energy harvesting condition, we changed the external load resistance: altering how much electricity was being generated and how much the carried mass would oscillate. We then correlated device behaviour to the biomechanical response of the user. The energy harvesting load carriage system generated electricity at no additional metabolic cost to the user, when compared to walking with the carried weight rigidly fixed. The device was able to generate up to 0.22±0.03 W of electricity, while walking with 9 kg of carried weight. The device also reduced the interaction forces experienced by the user, in the medial-lateral direction, compared to walking with the device when the mass was rigidly fixed.