Electromagnetic (EM) RF (radio frequency) energy harvesting in dynamic ambient environments is a challenge for conventional energy harvesting systems such as rectennas. The main challenges are the low efficiency of the collector and low ambient power levels, which makes it hard to consider in industrial applications. Several research works have focused on the design of high-efficiency antennas to achieve an efficient and maximum possible level of RF EM energy harvesting. Their main objective is to improve the EM energy harvesting system by overcoming the low efficiency of the collector, which is the main part of the rectenna system. In this work, we propose and investigate a methodology in terms of EM energy harvesting based on the concentration and focusing of EM energy in a small zone where it can be easily collected and transferred indirectly to the rectenna system. It consists of a focusing device and a methodology to associate the latter with existing RF energy harvesting systems. We demonstrate a focusing metasurface design implemented alongside an off-the-shelf rectenna device at 900 MHz, where an enhanced energy harvested power level up to a linear gain of 8 is achieved compared to the case when only the rectenna is used. Numerical results as well as measurements results in an anechoic chamber are shown. Experimental power received levels are given both in the focusing plane and in time for the validation of the concept.