The enerjar.

[Locut0s]

http://enerjar.net/

As they admit it's a bit of a dangerous hack but quite a neat little idea.

 

[Nohr]

That reminds me of Iron Man.

 

[Cheesetogo]

You can get a Kill-A-Watt for $20 on amazon, so unless you really want to build it yourself, I'd say buying one is the better way to go.