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What is a watt?

\mathrm{W = \frac{J}{s} = \frac{N\cdot m}{s} = \frac{kg\cdot m^2}{s^3}}


Just kidding, we don't want to bore you with fundamentals of electricity regarding watts, volts, amperage, etc. You can turn to Wikipedia for things like that or thousands of other sources.

What we wanted to try and do is write a very simple article about some of the questions we constantly get asked regarding AC adapters and charging capabilities. We'll try to teach you how to buy appropriate AC adapters to maximize your charging speed and compatibility.

When shopping for AC adapters for mobile devices, there are typically three key figures to look at. Volts, Amps and Watts. If you multiple Volts x Amps, you end up with Watts. The higher the wattage, the more power that is basically flowing through to your device. So a 1 Volt x 5 amp device is basically equivalent to a 5V x 1A device. A 5V x 2A charger delivers 10W or twice the electricity. Most AC adapters are based on 5V so the focus shifts to comparing Amps.

In addition to the output wattage, your device also has build in limits. For instance, the iPhone 5 has a maximum input of 5Vx1A. So even is your AC adapter will output 2A, the phone itself would limit the power draw to just 1A. However, with the iPhone 6 and 6+, they are capable of charging at 5Vx2.1A but Apple included only a 1A AC adapter. So using a 1A AC adapter, your iPhone 6 will charge at 1A rather than the full potential of 2.1A. It is perfectly safe to plug in a device with a lower amperage limit than the AC adapter. (i.e. plugging in an iPhone 5 with 1A max to a 2A AC adapter) as the phone itself limits the input. In addition to raw amperage limiting by a phone, some devices add their own unique limiting based on how it detects power coming in. For instance, we test many AC adapters that offer 2.4A maximum output and will charge Android devices at 2.4A but Apple devices technically capable of 2.1A charging will limit the rate to 1A. This has little to do with electrical limitations but rather safety protocols employed by various brands. When we develop AC adapters, we always make sure the maximum specs are reached with both Android, Apple and other devices.

Here is a simple hypothetical example using a 2A capable phone:

  • Using 2A AC Adapter = 3 hours to charge
  • Using 1A AC Adapter = 6 hours to charge
  • Using 0.5A AC Adapter (or computer USB port) = 12 hours to charge

There are some dynamics that make real life not quite so linear. For instance, batteries are typically programmed to charge faster when near depletion and enter a trickle mode to slowly top off. This helps prolong battery life.

So as a rule of thumb, the AC adapter can exceed the device to maximize your charging time.

So based on all this, one might think, "I should just get the highest amps or watts possible." That is somewhat true but with high wattage come some tradeoffs. The adapter size has to be physically larger. Heat is also greater. While we may use a beefy 4.8A four port USB hub in the office, we don't want to lug it around while traveling.

A quick note about dual USB chargers. The more common type of adapter will typically specify one port as 1A and the second port as 2.4A. So combined, this would be 3.4A. This is ideal for charging a 1A phone and a 2.1-2.4A tablet concurrently. Better designs of dual port AC adapters will have the same limit of 3.4A but use a "shared" approach so you can plug the tablet into either USB port. This is much more convenient than trying to remember which port was the faster port. Another iteration of this is a shared 2.4A dual USB. To us, this is the best of both worlds. You are able to carry a very small AC Adapter with dual ports. When you charge a tablet, you get 2.4A of output. Or on occasion, if you plug in two devices, they will each output about 1.2A. This may charge a bit slower if you have a 2A device but you benefit from having a small compact USB device. This is ideal if you only occasionally use both ports concurrently.

We hope we didn't add to your confusion. To summarize, the AC adapter should at least match or exceed the watts (Volts x Amps) of the device your are charging. If the AC adapter has higher watts than the device, that is safe and charging will be at the maximum capability of the plugged in device. If the AC adapter has lower watts than the device, that is still safe but will reduce the time it takes to recharge proportionally. Get the highest wattage charger you can that fits the form factor you desire, your charging habits and number of devices.

There are some interesting developments that are pushing the boundaries of charging speed. We'll write about this in another article as compatibility is more challenging. For instance, the Samsung S6 Edge features what they call "Adaptive Fast Charging" which is basically a 9V x 1.67A = 15W AC adapter. (charges 50% faster than a 5Vx2A=10W adapter). The same adapter reverts to 5Vx2A with other devices. Qualcomm Quick Charge is another charging standard that matches the adapter to supported devices and allows a maximum charge of 9Vx2A = 18W (80% faster than 5Vx2A=10W). It is important to note that both the charger and device need to support the charging protocol to ensure maximum charging speed. Then there is USB-C which technically can support up to 100W!

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