Do your windows get fill sun light? You could make a solar window heater thats like a black box that ducts into the window and the sun heats the interior and either convection or use a small solar panel and fan to move it.
FYI, in my 822 sq ft condo with all electric and heat pump I tried using a space heater in the room I was in only on low and then the whole house heat pump. It was cheaper to keep the whole place warm with the heat pump vs just me and the room I was in 16 hours a day.
I always cut back the heat or ac when Im not there.