MacOS – Super high network usage OS X Yosemite

I don't think that the total displayed in Activity Monitor is the total you think it is. I think it is maybe just the last online action a process had. I noticed that while watching softwareupdate download data.

Just to be sure: Is automatic software update disabled? See System Preferences, AppStore. Also, do you have anything set to sync itself? iCloud (Photos and Co.?), AppStore, iTunes, stuff like that? For example, iTunes has its own "update settings" (see Preferences, last tab).

You can check in AppStore's "Updates" tab to see the recently installed updates.

Do you have control over the router? Maybe it has logs with timestamps so you could see if any other device used it or what else happened! Because you said that the Internet is expensive, it could be that someone else is using it.

I assume you have the WiFi secured and it is not public? Use the strongest encryption your router offers and I would change the password, just to be sure.

May be it's the software that comes with the wifi router / 3G - have you installed anything for that?

Is this happening since beginning or started it later?

Also, you could ask your provider for logs or more information about this traffic.

Good Luck!

As far as I am concerned kernel_task was always trying to tell me that the computer is running hot. It seems to be a strange countermeasure which occupies the CPU with a low energy No-Op task which prevents it from doing "hotter" things. So you should be looking for that "other" process eating up the rest of your CPU time.

However I noticed very high CPU usage of Safari ever since I update do Yosemite 10.10.4. The browser visibly (in Activity Monitor) starts several processes now and two of them (Safari Networking and nsurlstoraged) routinely take up a lot of CPU, especially Safari Networking often takes more than 50% of one CPU. It seems to be less constant now, so maybe there is some "trickle down" effect on this one.