Recently I was curious about this. Since Macbook Retina 12" use USB Typc-C interface to charge, and the rated supply voltage is 14.5V 2A (USB-PD) or 5.2V 2.4A (clearly output voltage is a little higher for cable compensation),and Apple claims "Your MacBook will charge from USB-C power adapters not manufactured by Apple if they adhere to the USB Power Delivery specification." on this page. So I wondered, whether I can use a normal USB charger plus USB Type-A to USB Type-C adapter to charge MacBook Retina 12" or not.
Apple USB-C 29W charger. Output: 14.5V@2A(USB-PD) or 5.2V@2.4A
Image Source: ChongDianTou.com
Image Source: ChongDianTou.com
Since Apple won't tell you the technical detail of Mac, so there must be some brave guys to test for us. Luckily I found some tests on the Internet.
TL;DR: You can charge Macbook using normal USB charger.
WARNING: Compatible does NOT mean reliable or safety. You should use a USB-PD compatible charger with enough rated output power to charge your Macbook Retina 12" (e.g. Google Universal USB-C 60W Charger). Charging Macbook Retina 12" using a low power-output charger may result in over-heat in the charger, even fire. Charging Macbook Retina 12" using a fake/conterfeit or low-quality charger may result in danger of electric shock or fire or permanent damage to your MAC. I TAKE NO RESPONSIBILITY OF ANY POTENTIAL DAMAGE OR ANY INCORRECT IMFORMATION IN ANY CASE. TRY IT AT YOUR OWN RISK.TEST ONE from notquitemainstream.com (English)
You can use Google Universal USB-C 60W charger to charge your Macbook, but you will NOT benefit from a larger rated power (60W), since Macbook only takes 29W.You can use Apple USB-C 29W charger to charge Google Chromebook Pixel, but you WILL suffer a lower charing speed, since charger can only output 29W of power.
Right: Apple USB-C 29W Charger compatile with USB-PD.
TEST TWO from ChongDianTou.com (Chinese)
You can use a 3rd party Standard 5V USB charger to charge your Macbook Retina 12", but you WILL suffer a lower charing speed.An intersting fact was showed. While your Mac is in NORMAL CHARGE mode, the charge icon is different from that while charging in SLOW MODE.
Charging using offical USB-PD compatible USB-C 29W charger.
Since power conversion efficiency is lower than 100%, so input power is 31.8W.
Charging using a 3rd party USB 5V 2.1A charger with USB-A & USB-C ports.
(USB-PD not compatible, JUST normal charger plus a USB-C port)
Macbook cannot determine this 3rd charger, so it take only 5Vx1A=5W.
5V 1A is kind of standard spec, in order to insure your safety.
The power efficiency is lower than 100%, so input power is 6.58W.
Here is Mac icon for NORMAL CHARGING and SLOW CHARGING.
TEST THREE from bbs.feng.com (Chinese)
This brave man try using iPhone or iPad charger to charge his/her Macbook Retina 12". It turns out that you can charge Macbook Retina 12" with a normal USB charger which is not compatible with USB-PD.
Charge Macbook 12" using iPad 5.1V 2.1A charger.
Since power conversion efficiency is lower than 100%, so input power is 13.8W.
Charge Macbook 12" using iPhone 5.1V 1A charger.
Strange enough Macbook only take 5V 500mA. Maybe the iPhone charger is too old for her to recognize.
Since power conversion efficiency is lower than 100%, so input power is 3.2W.
Anyway this power is too low, and dfinitely will not effectively charge Macbook while OS X is still running.
CONCLUSION: USB WILL ALWAYS COMPATIBLE WITH 5V FOR BACKWARD COMPATIBILITY
No matter what USB will advance in the future, for safety, for backward compatibility, USB will always compatible with 5V. Remember if USB output a voltage other than 5V or a current higher than 1A, the host and client must comply with some sort of standard, like USB Battery Charging, or USB Power Delivery, or Qualcomm Quick charge 1.0/2.0/3.0. THE INITIAL AND DEFAULT PROFILE WILL ALWAYS BE 5V. If it is not designed like that, you can easily burn your device into fire. Of course, old charger rated with lower outut power, and you should not use a low-power charger to charge a power-demanding device in the normal case. But imagine, that you didn't bring your charger along with you, and don't worry, you can charge it using a normal USB charger to help you out. That's wonderful. Also, you can buy a new full-spec high-output-power universal USB charger for any USB devices.
Of course you are recommended to use official charger.
If you want use any other charger, you should know what you are doing.
IT IS A BIG STEP, THAT ONE PORT WILL SUPPORT ANYTHING!
Not far in the future, you will need just a single full-spec USB charger to charge your notebooks, mobile phones, tablets, power banks, and more.
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