A new study conducted by Colorado State University (CSU) and commissioned by the Wireless Power Consortium (WPC), which is behind the Qi standard for wireless power, showed that Qi's inductive charging is more efficient in terms of energy use than the competing standard, Rezence.
Rezence, a wireless power standard that uses resonance charging, can cover a wider area for charging devices, such as an entire table surface. Qi, on the other hand, can only charge a device in a fixed location. The Rezence standard is now being developed by the Alliance For Wireless Power (A4WP). The consortium recently merged with the Power Matters Alliance, the creators of Powermat, to better compete against the fast-growing Qi standard.
The battle between WPC's Qi standard for inductive charging and A4WP's Rezence for resonant charging hasn't yet been settled. In fact, some companies have even started using them together rather than picking between them. Right now, at least, the two standards are also more complementary rather than directly competitive, so it makes some sense to use them together, unless the licensing costs are prohibitive for most manufacturers.
The A4WP called Rezence the next generation of wireless power because it can charge devices that are within a short distance. However, Qi has at least one significant advantage compared to Rezence, and that's efficiency in power usage.
Colorado State University tried to find a more standardized way to measure efficiency for both Qi and Rezence wireless charging systems that isn't biased towards either one of them. Therefore, it came to the conclusion that the efficiency should be calculated as follows: "DC power into the battery divided by DC power into the transmitter with a 5W battery load (4.2V at 1.2A)."
The University believed that this would be the optimal way to measure efficiency because it reflects the power needs of a smartphone, which is currently the typical use case for wireless charging.
In the test, CSU researchers tested a commonly-used A11 transmitter coil for Qi charging. For Rezence, it tested a system from Efficient Power Conversion (EPC), which uses Gallium-Nitride output switches, a zero-voltage switching (ZVS) output topology and the NuCurrent antenna set compliant to the A4WP class-3 spec -- all of these giving maximum efficiency as far as a Rezence system goes, according to the WPC.
The test results showed that in most cases where the transmitter and receiver are in close proximity, such as having charging pads on which to rest your smartphone, Qi charging is optimal as it provides the highest efficiency (and therefore the fastest charging time).
Rezence can lose more than 800mW of power due to its high-operating frequency. However, the WPC also said that it may be appropriate sometimes to sacrifice some energy efficiency to reach a greater distance. That's why some of the Qi backers are also backing Rezence.
Although smartphones may not necessarily need Rezence charging, it could be significantly more convenient, for instance, to charge electric cars this way in the future. All parking spots could be powered by Rezence, or perhaps an even longer-distance wireless charging technology, and the cars would just need to park and be charged at the same time.
We should also note that a study commissioned by one group that proves their superiority in any way to a competitor is to be taken with a grain of salt.