3. Efficiency

The efficiency of wireless power is the ratio between power that reaches the receiver and the power supplied from the transmitter. Generally wirelessly transmitted energy is dispersed as the energy radiates into the environment or is lost as heat at the transmitter or receiver. Wired transmission techniques on the other hand lose far less power as wires are good conductors and help to confine and guide the energy to where it is needed. Generally, wireless energy transfer works best at short range; although long distances are possible if the transmitters and receivers are physically large, or the energy is able to be formed into a tight beam, such as with lasers or large microwave dishes. The ultimate angle subtended by a beam is limited by diffraction.

When phased arrays are used for wireless transmission, the phased array normally needs to be contiguous due to a phenomenon called the thinned array curse; gaps in the array act as a diffraction grating and causes side bands that lose energy.

Microwave power beaming often achieves higher conversion efficiency than lasers, and is less prone to atmospheric attenuation. However microwaves have far longer wavelengths than visible light, and require proportionately larger transmitters and receivers to deal with diffraction, particularly over long distances. The most efficient laser power beaming system today has photovoltaic panels optimized to the wavelength of the laser. Losses due to atmospheric spreading can be reduced by the use of adaptive optics, and losses due to absorption can be reduced by a properly chosen laser wavelength. Laser power beaming does not work well through clouds.

Although laser and photovoltaic technologies have been rapidly advancing, it is unknown what transmission efficiency improvement is possible. The most efficient lasers — laser diode arrays, can surpass 50% efficiency, but such lasers do not have mutual coherence. Other options include standard chemical lasers with efficiencies of a few percent or less. High-coherence diode laser arrays or a similar technology would allow for notably improved power usage efficiency, as laser inefficiency comprises most of the energy loss.

Taking the theoretical example of transferring 50 MJ of energy from one place to another (see space elevator and space elevator economics): The base cost of payload transfer, given the current power grid rate of about US$0.11/kW·h = about US$0.03/MJ, is around US$1.74/kg. Factoring for transmission losses, assuming current laser efficiencies of 2%, solar cell efficiencies of 30%, and atmospheric losses of about 20%, this works out to about 0.5% overall efficiency, or $350/kg.