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Wireless sensor networks (WSNs) have many potential applications for both military and civilian use. Volume and weight of the nodes are important parameters which can limit the usability of a WSN. Today the electronics and sensor systems utilizing MEMS technology can be made very small and robust. However, the power system which ideally should be only a fraction of the node volume will constitute a dominant part of the volume in order to get an acceptable node lifetime. Harvesting energy from the sun is the most effective technology for outdoor applications. A combination of solar cells and chargeable batteries gives nodes with potentially eternal lifetime. The sun irradiance varies through the day and through the seasons, and is strongly dependent on the latitude. This must be taken into account if a wireless sensor network should be powered with solar cells at high latitude locations. The PVGIS database gives geographical assessment of solar resource and performance of photovoltaic technology in Europe. We have studied how much energy can be harvested for a credit card sized Si solar panel at six different latitudes in Europe. With a typical WSN node and with calculations from the database we show that it is difficult to get acceptable results during the winter months. At 50° and 60° north latitude the sleep / active ratio is approximately 500 and 1500 respectively. North of the Arctic Circle the system will not work at all. During the summer months there is very little difference in irradiance since the increased inclination at high latitude is compensated with longer days. This is with standard electronics. Using state of the art electronics with much higher efficiency sleep/active ratio can be increased four times.