Probably not many watts required to keep temperature. It is a vacuum. The filament in a vacuum tube glows with very few watts.
My concern centered on whether surface tension and lack of gravity would make the solder ball up. Yes, adhesion and surface tension also play into the spread of molten solder and its wetting of the parts, but has soldering actually been tested in space? I have done overhead soldering for plumbing, but I am not sure that is a good model for microgravity.
John