In previous posts we learned about room-scale wireless power and looked at some real world deployments of this innovative technology. In this post we will get technical and review the first key specification of such systems.
Not surprisingly, this most important metric is Power. Specifically:
Client Power — power the client device can use.
Before we dive in, let’s quickly review the key parts of a wireless power system:
Transmitter — beams energy. The beams can be radio waves, invisible light, other electro-magnetic fields, and even acoustic energy.
Receiver — receives energy, converts it to electricity, and delivers it to the client device.
Client device — uses the electrical energy. This can be your phone, toothbrush, gaming controller, etc.
An invisible part of the system is the path between the transmitter and receiver. In other words, the space of the room in which the wireless power system is installed.
The above may sound familiar to how our phones and other devices communicate. Communication goes both ways, but to keep it simple imagine the cell tower as the transmitter, your phone’s antenna as the receiver, and your phone as the client device. The path is everything in between.
Client Power vs Transmitted Power
You won’t be surprised to read that not all power beamed by the transmitter becomes client power. Some of it is lost along the path, and some is lost inside the receiver as part of the conversion to electricity.
Beware of systems that specify only the power beamed by the transmitter and do not disclose client power. That’s like getting paid without knowing the tax rate. With some wireless power technologies the taxes can get exorbitant.
How much power do I need?
Fuel pumps deliver gallons, Wi-Fi delivers megabytes, and power systems deliver Watts.
Let’s review some familiar devices and look at ballpark numbers.
Smoke detectors, key fobs and TV remote controls typically consume less, or even much less, than 1 milliwatt (mW, one-thousandth of a Watt).
These ultra low power devices are so simple that they can last years between battery replacements.
Electric toothbrushes, smart door locks and doorbells, thermostats, Wi-Fi security cameras, and most other smart devices, consume on average about 1 to 10mW. These usually last a couple weeks to a few months on a single battery charge.
But, this low power consumption is achieved by the devices being almost always off. A Wi-Fi security camera records short videos only when movement is detected, and a smart door lock unlocks a door only a few times a day.
Your phone or smartwatch usually needs to be charged daily. That comes to about 50 to 500mW. Interestingly, these are the only devices that are important enough for us to go through this bother.
The next class of devices are those that must always be wired, or allow only a few hours of wireless operation at best. Laptops consume 20 to 70W and require a bulky battery to last through a few hours away from a power cord. With TVs and other appliances we get to 100W and above, with batteries no longer an option.
Modern wireless systems focus on the middle two categories and provide power in the 1 to 1,000mW range. The <1mW devices last so long on batteries that wireless power is not needed. The higher power devices can be powered wirelessly, but that stage of the technology is not yet mature.
Summary
The bottom line of any wireless power system is Client Power — how much power the client device can actually use.
A minimally useful wireless power system must provide at least 10mW of client power — enough to power a single, simple, smart device.
A truly useful system should deliver 100–500mW. This allows powering multiple devices, enhancing their functionality, and powering more hungry devices that are typically wired or charged daily.
Providing a lot of power is a great start, but power alone is not enough. Just like with Wi-Fi or cell signal, you may get far less bars showing on your phone than you hoped to.
In our next post we will discuss this and explain the second key metric of a wireless power system — coverage.
