Actually thezipper is right, it would be either in the Lav (which the other ~400 customers will not appreciate you spending a couple hours in there with your laptop), or in a WBC seat. Other options would be to bring spare laptop batteries along with you. Depending upon your circumstances and fare class, you might be eligible (Y, B, M fares) to use miles or PMU to upgrade to WBC (depending upon availablity of upgrade space) to WBC if powering your laptop is critical during the flight.

There is essentially no chance that you would be allowed to put your laptop onto charge in an empty WBC seat (if there even was one).