At minimum I'd suggest getting something to insulate them from the window sill, this could be PIR board or a silicone mat, but the heat mats I've used in the past have been very effective, the ones I've used are about 18w, which isn't a huge power draw.
No clue how cold it gets where you are, but the tomatoes and beans certainly seem like they'd benefit.
Heating lamps are effectively a waste of power.