To measure physical activity in a wheelchair user is important. Disabled people have been found to be less active and it has negative health implications. By relating to ones own physical activity in concrete form it is easier to reinforce a positive attitude towards being physically active. The aim of this thesis is to show how a system can be built, that is able to collect measurements from sensors, with usability in mind. The system consists of software that integrates an iPhone, Apple Watch and a Tacx rotation sensor mounted on a wheelchair. With the help of a physics model these measurements can estimate energy expenditure of a wheelchair user. The system was tested with four participants performing subjectively decided low, moderate and vigorous physical activity while the system collected data. In terms of absolute energy expenditure the system was inaccurate. On the other hand, correlations were found between the calculated energy expenditures and physical activity. Using measurements from the rotations sensor a correlation of 0.7315 was found between the calculated energy expenditure and physical activity, a correlation of 0.5385 using measurements from the GPS and a correlation of 0.6452 using measurements from Apples own proprietary algorithm. The created system is a promising prototype towards objective measurement of physical activity in wheelchair users.