This work investigates the possibility of improving the economic performance of PV in Northern Europe. The PV system investigated in this study is assumed to be connected to the electricity grid. However, because there is a difference between the electricity tariff and feed-in electricity price, adding a battery to such a system improves its economic viability. But the application of a battery increases initial costs of the system. Therefore, it is especially of economic relevance to choose the optimal size of the battery. In this work, the optimal battery size was calculated under different parameters including electricity tariff, feed-in electricity price, and battery performance and price. For this purpose, an actual building located in Landskrona, Sweden was chosen as a case study. A computer model was built to simulate the PV system. The simulations show that the optimal battery size is strongly influenced by the local conditions especially the feed-in electricity price.