Komatsu Forest AB in Umeå saw an opportunity to increase performance during fast varying loads, e.g., when starting to feed after the felling cut, that proved more difficult to reach with their new line of stage V forest harvesters. It was believed that this issue was magnified as a result of new environmental regulations which limited their diesel engines' fuel injection quantity with low turbo boost pressure. It was in Komatsu Forest's interest to make comprehensive measurements of hydraulic and engine parameters in order to investigate what was limiting performance. This thesis includes a literature study into a number of viable options for storage types for a heavy hybrid vehicle with the purpose to recommend the best suited type for Komatsu's harvesters. It also includes extensive measurements made on a producing harvester where hydraulic and engine parameters as well as for example valve currents were logged and used as grounds in making the energy storage recommendation. With results from the measurements, the goal was to estimate how much energy is needed, how much energy can be stored and lastly to design an energy storage to supply additional power to increase performance. With successful measurements, it was possible to detect that approximately five kJ of energy with a maximum momentary power of 37 kW was sufficient to achieve a smoother first feeding. Furthermore, approximately 9-14 kJ of energy and between 0,3 and 0,5 l of hydraulic fluid was estimated as potential stored energy and volume from feeding stops of different sized trees plus an additional 3 kJ and 0,1 l of fluid from cutting stops. The best suited energy storage then proved to be an accumulator mounted on the P-line, which resulted in an appropriate accumulator volume of 4 l with an estimated pressure range of 250-280 bar.