Wireless power
tl;dr:
- not very much power
- not very long distance
- only moderately efficient, lessened with range
As such,
- it's not used on beefier devices,
- it has mostly ended up as a convenience e.g. in mobile devices.
There are three major variants:
- Wireless Power Consortium (WPC)
- e.g. Qi
- frequency: ~110-200kHz
- power: order of 5 Watts (planned higher) (verify)
- inductive
- Power Matters Alliance (PMA)
- frequency: ~300kHz (277 kHz to 357 kHz?)
- power: 3.5W to 15W, planned higher? (verify)
- inductive
- Alliance for Wireless Power (A4WP)
- e.g. Rezence
- frequency: ~6.6MHz
- power: order of 5 Watts (planned higher)
- range: up to 5 cm (for decent efficiency; can work at more), allowing e.g. under-desk mount
- resonance
On range
Range of each of these may be mentioned as 10 meters.
However, this is a "you can maybe tell a transmission device is nearby" range. If it manages to transfer anything at all, it most likely has absolutely terrible efficiency. You should assume it needs to be nearly touching to be halfway okay efficiency.
On efficiency
It turns out there is usually a very real difference between
- ideal hardware in lab conditions - Minimal distance. Expensive hardware. Good shielding. Good alignment of transmitter and receiver.
- ...and what you will see in reality.
Assume that that quoted figure from lab conditions, transported to a real-world device, will actually be noticeably less.
Real-world data (including some official graphs from manufacturers) as well as amateur tests (e.g. charging a phone wired versus wireless) suggests efficiency peaks out at 60% to maybe 70%, which decreases with distance, poor alignment, and cheap designs.
From an engineering standpoint, that's peak is actually pretty decent.
From an efficienty standpoint, you are typically using ~40% more power compared to just plugging it in in a good case, and maybe twice in a bad case.