Well, there's 3 ways to do it- a capacitive voltage doubler, a transformer, or an inductor-based DC/DC converter.
The cap doubler has many things going for it, it's theoretically lightweight, simple, and doesn't draw much idle current to stay on with no load. However I have trouble picturing the capacitors which could handle 50W. That's over 8 amps ripple current (4 amps in + 4 amps out running through the cap). Even most tantalums have ratings like 100 mA or so.
The inductor would have to be quite large, a transformer would be a better choice.