The emerging vast acceptance of autonomous electronic devices (e.g. radios, cell-phones, laptops, robots), which are currently being powered by on-site conversion of chemical energy, justifies revisiting the old dream of the pioneers of electrical applications (e.g. Tesla, Edison): transporting electrical energy wirelessly; where for optimal practicality, the energy transfer should be independent of the details of the geometry of the space in which the scheme is being used (e.g. of the exact position of the device with respect to the source, and whether there exists a direct line-of-sight between the device and the source.) Of course it is well known that freely-radiative modes satisfy this requirement (making them very suitable for information transfer), but they are not suitable for powering remote devices, since most of the power ends up being wasted into empty space. In our work, we investigate whether, and to what extent, the unique physical phenomenon of long lifetime resonant electro-magnetic states can, with long-tailed (non-radiative) modes, be used for efficient energy transfer. Intuitively, if both the device and the source are resonant states of the same frequency with long lifetimes, they should be able to exchange energy very efficiently, while interaction with other environmental off-resonant objects could be negligible. Of course, intricacies of the real world make this simple picture significantly more complex. Nevertheless, via detailed theoretical, and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme could indeed be practical for middle-range wireless energy transfer (i.e. within a room, or a factory pavilion). Important novel applications are thus enabled.