More recent in-seat sockets will have a USB slot as well as a two-pin that will take both European and American plugs, and sometimes UK and Australian ones.

Don't expect to be charging flat batteries - these are for an often intermittent trickle feed that will allow you to operate your device but probably not charge it. I've been on a couple of flights where people attempting to charge hungry batteries have caused the whole power feed to shut down.

If you're running a laptop, best to remove the battery if possible.