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How efficient is wireless power?
The energy consumption of battery chargers has two main contributors: charging efficiency and standby power consumption.
STANDBY POWER CONSUMPTION Unfortunately, many people leave the chargers and cradles connected to mains power when the charger is not used. The standby power consumption (also called “no-load power consumption”) is significant. A simple calculation shows that power consumed in standby mode is about the same as the energy consumed when loading the battery.
We assume that many people will also keep their wireless battery chargers continuously plugged into the mains. One of our main design goals was, therefore, minimize standby power. Go low!
We did go low. In the mean time we have demonstrated a system with only 0.0001 Watt (100 µW) standby power consumption. And that is probably not the bottom.
CHARGING EFFICIENCYThe other contributor is charging efficiency. Our wireless chargers have the same ingredients as a wired charger (an AC-DC adaptor plus charging electronics) and one additional ingredient: the copper wire between adaptor and the mobile phone is replaced with a wireless link. That link is not efficient as a copper wire (what can beat a copper wire?), but careful design made it possible to achieve at least 70% transfer efficiency. And that percentage can go up a bit if a manufacturer is willing to spend more on high-quality components.
TOTAL ENERGY CONSUMPTIONA wireless power transmitter can be more efficient, or less efficient than the wired chargers it replaces. It depends on the number of wired chargers that are replaced. It also depends on the type of chargers, and on the habits of the owner.
We estimate that in typical situations a wireless power charger breaks even with wired chargers if you replace two wired chargers. Details of that calculation are here.
An estimate of power consumption by wireless chargers.
POWER CONSUMPTION OF WIRED CHARGERS
Let’s first look at the power consumption of a classic mobile phone charger. These chargers are simple so-called “external power adapters”. A good source for data is the ENERGY STAR website. Here you will see that Energy Start compliant AC-DC adapters typically rate:
- Efficiency @ max load: 72% on average for 5 Watt adaptors
- Power consumption @ no load: 0.12W on average for 5 Watt adapters with a few exceptionally good adapters going down to 0.01 W.
Suppose that you use the adapter for 1 hour per day, and that it remains plugged in for the rest of the day. That is not a good practice, but it is quite common to leave power adapters and cradles continuously connected to the mains.
You see that the total energy consumption is:
- charging: 1 hour * 2 W / 72% = 2.8 Wh (this assumes that 5 W charger will supply, on average, 2 W during a complete charging cycle)
- standby (no load): 23 hours * 0.12 W = 2.8 Wh
You see that standby power contributes significantly to the total energy consumption of a mobile phone charger.
WHAT ABOUT WIRELESS CHARGERS?Our wireless chargers also contain an AC-DC power adapter. Let’s assume that is has the same efficiency (72%). Let’s also assumes that it has the same standby power (0.12 W). [footnote: Wireless chargers can have a much lower standby power, but this keeps the comparison easier.] The transfer efficiency of the wireless power link is typically 70%. And assume that the wireless charger replaces 2 wired chargers. The total energy consumption is:
- charging: 1 hours * 4 W / 72% / 70% = 7.9 Wh (we are now charging 2 devices simultaneously)
- standby (no load): 23 hours * 0.12 W = 2.8 Wh
HOW DOES THAT COMPARE WITH THE WIRED CHARGERS?Total power consumption of two wired chargers: 2 * ( 2.8 + 2.8 ) = 11.2 Wh
Total power consumption of one wireless charger with two receivers: 7.9 + 2.8 = 10.7 Wh
You see that the total energy consumption is comparable. Although wireless transfer is obviously not as efficient as transport over a copper wire, wireless power transmitters saves standby power energy when the wireless transmitter replaces multiple external power adapters. |
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