Abstract—As battery capacities are a key limiting factor of wireless sensor networks, harvesting energy from the environ- ment is very attractive. For outdoor applications, solar power seems to be the best suited energy source. However, the amount of energy delivered from the sun changes significantly over the year, which makes,the dimensioning,of the panel difficult. In this paper we discuss the ... [Show full abstract] most,important,impact,factors and,introduce a model,that predicts the harvested solar power,and the battery charge over the year. In addition, we present experimental results of the first six month,of our long term experiments,for validating our model.