According to Treffers, charging capability "drops off a cliff" beyond a distance equal to the size of the coil. Power by Proxi's Cross agrees, and even said anyone trying to sell a room full of wireless power or saying it will be as ubiquitous as WiFi is being "dishonest."
"We don't think it's an efficient use of power or a safe use of power," Cross said. "We don't think it will be possible for a whole bunch of reasons: the size of the coils required to broadcast power at long distances, the levels of efficiency will quickly drop down to 10% or less. You can broadcast wireless power over six feet, but the amount of power received will be less than 10%."
Schatz, however, scoffed at naysayers.
For example, another of WiTricity's charging kits for developers -- the WiT-2000 -- is designed to transfer energy over distances ranging from less than a centimeter (directly on the surface of the charging pad) to distances totaling several tens of centimeters.
"We can demonstrate where a 10 centimeter coil can operate over 20, 30, 40 centimeters," Schatz said. "He is correct that when things are closer together they can be a bit more efficient, but they can still be efficient enough and very useful when they're at a longer distance."