Macbook/Laptop: USB-C Power Banks Explained

Portable Charging Can Be Tricky

With so much jargon used to describe power and power banks, it can be tricky finding a device to meet your needs. The purpose of this FAQ is to demystify those pesky power bank specs. 


How Are Volts, Amps, and Watts Related?

Think of power transfer and charging as a garden hose filling a kiddie pool.

Voltage (V) in our analogy is the diameter of the hose.

Amperage (I) is the measurement of current, or in our analogy amps are the water-pressure.

The diameter and water-pressure are what determine the flow rate. If we were filling a kiddie pool and have a large hose with low pressure, or a small hose with higher pressure, we may have the same filling rate. The product of voltage and current is power and is measured in watts.


An example of the wattage equation?

The equation is:

Volts X Amps = Watts

An example of this is in light bulbs:

The standard American household runs 120V AC. Using the wattage equation we can solve for a number of amps drawn by a standard 60-watt bulb.

120V X Amps = 60W

Amps = 60/120

Amps = .5

We find that a 60-watt bulb will draw about half an amp.

What are watt-hours?

Watt-hours are a measure of power bank capacity. One watt-hour is equal to the amount of power a single watt will draw over one hour.

An application of Wh would be how a 60Wh battery powering a 60-watt light would work for around an hour (depending on inefficiencies). Think of watt-hours as a common currency, consistent across all platforms.

Is mAh even useful?

Sometimes manufacturers do not include specific watt-hours and only mention the milliampere-hours (mAh), but the mAh won’t actually tell you how well the power bank will charge your device. Watt hours is equal across devices and power banks (though affected by charging inefficiencies).

You can convert from mAh to Wh by taking the milliampere-hours x volts/1000.

Will too high of amperage damage my device?

All early USB applications were 5v, so the mAh of a battery was a good indicator of its relative performance. Now with USB C and the 3.1 power specs, a power bank can work at much higher voltages.

Think of it this way: A 5-volt by 4-amp charger will charge a device twice as fast as a 5-volt by a 2-amp charger, but a 10-volt by 2-amp power supply would ruin the device.

And while too high of a voltage may damage a device, high amperage alone will not cause any issues. The device will draw the appropriate amount of amps, and the MacBook can function with very high voltages.