This thesis has its starting point in an existing computer model of an electricity generating kite, from Heidelberg University. The modelled kite has an area of 500 m2 and is tethered to a generator at sea. A control unit steers the kite in an optimised trajectory. The design and trajectory that maximise mean power output per loop had been found using the optimisation software MUSCOD-II. Firstly, the model is investigated in order to find possible adjustments to make it closer to reality. Then a method to take the economic aspect into account in the optimisation has been developed. The most important findings in the model survey concerned wind speed. The original model overrated the wind speed at high altitudes and it used a mean wind speed instead of including yearly variations. Adjustments are made and a new objective function aiming at maximising the yearly average power output per invested Euro is used. Furthermore, the revised model has a preset wind speed range within which the kite can operate, and a maximum power output of the generator (the nominal power) which is found through optimisation with respect of cost. Cable strength and other production limitations are included as well. Using cost estimations for relevant parts, the revised optimisation model results in a system with a tethering cable about half the original length, and a steadier power output over the loop. The yearly production sums up to 16.8 GWh, as compared to the original model which would have given 42.9 GWh yearly.