This thesis aims to contribute to improved functional procurement of photovoltaic systems, PV systems. Functional procurement means that criteria concerning a function are set rather than specifying technical details or products. Possible advantages and difficulties in using functional procurement have been analyzed by interviews, simulations and a case study. Electricity production and surplus production for different scenarios have been simulated via MATLAB. The MATLAB script was also used to generate rules of thumb on feasible solar fraction for six different load profiles; a church, a farm, a car mechanic, an office, an industry and an apartment building. The results show that functional procurement promotes innovative solutions and is likely to increase customer satisfaction. The entrepreneurs get increased responsibility for the PV installation and the monitoring of it. Difficulties in using functional procurement of PV systems include uncertainties in irradiance and division of responsibilities. It is therefore important to define in what circumstances the functions should be met. The evaluation of the case study shows that the demands that were set were hard to accomplish, but if the maximum simulated surplus production would have been 6 % instead of 5 % of the summer weekdays, it would have been easier. The rules of thumb show that depending on load profile and accepted proportion of surplus production, the yearly solar fraction is between circa 10 % and 30 %. This number is lower for businesses that close during the summer and higher for businesses that have a load profile that corresponds well with the irradiance.