The development of autonomous battery powered systems which can be deployed in inaccessible locations for sensing applications has determined the development of various energy harvesting systems. Such an energy harvester is the one developed by Powercast which can convert the energy of radio frequency signals into useful power. A model of the harvested power can prove to be a useful tool for simulation purposes as it can provide, to some extent, prior knowledge of available energy resources when optimally deploying sensor networks. To obtain an accurate model of the harvested energy we have developed an experimental setup which has been used to determine the harvested power in two different environments, a hallway and a parking lot. We have developed the experimental setup to determine the amount of power available at the output of the radio frequency harvester which consists of a current measurement system and a data acquisition system. We have also modeled through simulations the harvested power based on the characteristics of the transmitter and receiver antennas and those of the environment. We have compared the results obtained through in field measurement with the ones obtained through simulation and we have shown that within certain margins of error of maximum 2 dBm one can successfully predict the amount of energy the system can harvest. However the RF-DC and Boost converter efficiency are also key factors in the quantity of harvested energy.