The power supplies in the seats are usually not very powerful.
Planes do have 120/240 plugs but usually not in Economy class.
Saving energy you can only do by shutting down things. WiFi, bluetooth. Minimum amount of apps running.
My solution during flights is fat powerbank.
I use the powerbank to keep my laptop charged and at the same time charge my powerbank with the USB outlet of my seat. That won't keep my powerbank fully charged but it surely helps a little. I just keep it plugged in all the time also when not using my laptop.
Works great on intercontinental flights.