Why is Charging Efficiency Important?

May. 2018

Everything else being equal, one would prefer to be efficient and not inefficient.

But with wireless charging, the end-to-end efficiency is even more important.

Wireless power systems have transmitters and receivers. The transmitter sends energy. The receiver makes it available to the device you want to charge or power. Measure how much electrical power goes into the transmitter. Measure how much electrical power comes out of the receiver. Comparing the these two numbers gives the end-to-end efficiency. A 1% efficiency means that 10 watts going into the transmitter generates 0.1 watt out of the receiver. 10% efficiency means that the same 10W generates 1W out of the receiver.

But why is that important? Let’s use an example to find out.

Let’s assume you want 2 watts to charge a phone. If your end-to-end efficiency is 10%, you need 20W into the transmitter. That’s like a modest energy-efficient LED light bulb. But if your end-to-end efficiency is 0.1%, you need 2000W into the transmitter to charge the phone. That’s like a blow dryer. Now, do you really want to have a blow dryer on for hours to charge your phone? I wouldn’t want to see your energy bill at the end of the month.

The other question is what happens to the remaining energy. Let’s assume 1% efficiency in charging the phone. 200 watts go into the transmitter. 2 watts go to the phone. Where do the other 198 watts go? Some might heat the transmitter or receiver. But the bigger concern is energy going into the room. We don’t want humans, pets or plants in the room to absorb them all the time.

When I was a kid my parents reminded me: not everything that can be done should be done. That’s also true today. If you had a link with 0.1% efficiency, you should not charge a phone even if you could do it in theory.