Re: Energy Efficiency
OK, let's get one thing straight from the start: This will always be an inefficient way to power a device. You are sending out a signal in all directions, wasting most of the power, and receiving a fraction of it somewhere.
However, when the router is transmitting for other purposes (e.g. what it was designed for, providing a WiFi signal)), this power would be lost anyway. Therefore it is an improvement to capture and use it. The problem comes when it is being powered outside it's normal use.
Now, in a normal domestic setting, the transmitter probably remains off for the vast majority of the time. So this scheme would likely be very inefficient. However, think of busier places, like office blocks, retail premises, transport hubs, distribution centres. All of these tend to have WiFi, and they will be running a hell of a lot more than your average domestic set up. The overall efficiency of it as a power distribution system will increase a hell of a lot. I don't have numbers, but I suspect that in a 24/7 operation with WiFi "constantly" in use and many small sensors dotted around, it could exceed 100% (i.e. most of the power is captured from wasted power in WiFi signals transmitted anyway).