What happens if I connect the MacBook to a USB-A port of another computer?

[Junichi]

So I bought a USB-A to USB-C cable with which you can charge your MacBook through an iPad charger or regular battery packs with USB-A port, and I'm wondering if I can use this cable to charge or power my MacBook from another computer.
I often use my MacBook in front of a school computer, and if I can power my MacBook from it like I always do for my iPhone and iPad, I don't have to carry the iPad charger.
Given the amount of power a USB-A port of a computer can provide versus the amount of power the MacBook consumes, I don't expect it to be able to really charge my MacBook, but it helps if at least it can power it and extend the battery life for an hour or two.
My concern is if it has any potential risk of damage to any of the parties (the MacBook, the school computer, or the cable).
Can I have your opinion?

 

[hardax]

I am really interested in knowing this as well.

I travel for work with my 15" Macbook pro and just got a 12" ordered this weekend. Wondering if I can charge the 12" via the USB port on the 15" that way I wont have to travel with 2 chargers.

 

[Frederik on MacRumors]

Assuming it is possible at all, and not blocked by some mechanism:
Apple says that their USB 3 ports output 900 mA. (https://support.apple.com/en-gb/HT204377) By googling, I found some sources that state a minimum of 1500 mA (or 1,5A). Hence, a MacBook can't be charged by a MacBook Pro.

Other notebooks or computers might supply enough power to charge a MacBook.

EDIT: USB 3 Specifications theoretically work with 1500 mA (https://en.m.wikipedia.org/wiki/USB#USB_3.0). Nevertheless, offering that option on a device does not make that much sense since it does not allow concurrent data transfer. Hence, I guess there is no way to charge a MacBook from another notebook...

EDIT2: Well... I guess it works anyway as can be read below...

 

[Cergman]

I've charged my 2015 rMB with my 2015 15" rMBP using a USB C to USB A cable when I'm too lazy to get its charger. Works fine, albeit slowly.

 

[Junichi]

When people say you can't charge a specific device with a specific USB charger, I'm pretty skeptical because oftentimes you actually can. Apple says you can't charge it when the charging rate is too slow.
I have seen a review that said he was able to charge his MacBook with a 5W iPhone wall charger very slowly, so it seems reasonable that a MacBook Pro's USB port can charge a MacBook.
However my concern is if it is generally safe since I don't really know how all those charging mechanisms work. I have my cable and my MacBook so I can always try and see if anything can charge my MacBook, but I just want a confirmation that it is safe to try.

Thanks

 

[violet ink]

I plugged my MacBook into my boss' MacBook Pro (Mid-2015 15") the other day, since I couldn't be bothered to plug in my charger on the floor, and it reported charging at 10 Watts.

Spoiler

 

[Junichi]

This is how the iPad Air wall charger works on MacBook if anybody is interested.
[doublepost=1480818070][/doublepost]Isn't it a little odd?
USB 3.0 port should be 900mA and 5V, which makes 4.5 Watts. How does make it 10 Watts?

I plugged my MacBook into my boss' MacBook Pro (Mid-2015 15") the other day, since I couldn't be bothered to plug in my charger on the floor, and it reported charging at 10 Watts.

Spoiler

 

[Weaselboy]

I plugged my MacBook into my boss' MacBook Pro (Mid-2015 15") the other day, since I couldn't be bothered to plug in my charger on the floor, and it reported charging at 10 Watts.

Spoiler

Here is a shot of violet ink after breaking the bosses MacBook.

Kidding... thanks for the test.

 

[Junichi]

Just connected my MacBook to an iMac. Curiously it gives 10 watts as violet ink reported about MacBook Pro.

 

[Junichi]

I tried a Windows desktop computer. It's probably USB 2.0.
From what I see here if you connect it to a regular USB port, you can use your MacBook for 40 hours. That's impressive.