Extend laptop battery performance?

I will be flying on a 5 hour flight and I would like to take my laptop with me to work on the plane. My battery life is about 3 hours, so I'm trying to decide whether I should buy a spare battery to take, or whether there's another charging method that I can bring with me to charge my laptop while on the plane. There are no empower port on the plane (at least it's not guaranteed). Any suggestions??