kernel_task cpu usage

Did you try to quit that Kernel task (which task is it?) in Activity Monitor?

 

Killing kernel_task isn't going to help. You'd have to do that from the command line but I'm betting that would pretty much immediately crash your system.
The problem has been around for years and I'm guessing the search function was still disabled when you posted this as there are a few threads discussing it on this board and many hits from Google.

What I've been able to determine - I have same problem - kernel_task is attempting to cool down the system by eating up CPU cycles so whatever is heating it up can't continue to do so. Sometimes it's related to battery versus wall power. Sometimes it's dust bunnies or fan slits being plugged..
In my case it's when I have a lot of Safari windows open or I'm transcoding video. Both those seem to be CPU intensive and I've learned to do one or the other. Good luck tracking down your cause. Searching this forum or Google should aid in that.

 

I had the very same issue and I eventually managed to figure out what was going on.

kernel_task is actually taking away CPU cycles when the CPU is running too hot. I determined that when I lifted the base of my MBP to allow for more air circulation the issue actually went away.

I bought myself a laptop cooler like this one:
http://www.amazon.co.uk/Coolermaster-Notepal-U3-Cooler-Notebook/dp/B00441LYEW/ref=sr_1_9?ie=UTF8&qid=1384717593&sr=8-9&keywords=laptop+cooler

and its been running happily ever since. I'm pretty sure it's using the Intel Turbo Boost on the CPU since its running cooler and can handle it more easily.