Empower on Airlines for HP Laptops?

I'm going to be flying on Japan Airlines soon and I learned they use EmPower on board the aircraft. Apart from the regular charger that was included with my HP dv4 laptop (American version, if any variation), I do not think I have any other adapters. My question is: What do I need, specifically, (and explain in simple details w/o using too many technical terms) to utilize the electricity so as to power my laptop aboard a long flight? How much will it cost me roughly? And I think you need something like a car adapter (cigarette) as well as an Empower adapter or a unit that contains both but I'm not entirely sure. People who fly regularly on planes that make use of EmPower, please enlighten me.