Passive solar heating worked remarkably well for me when I was living abroad in a place with so little electricity that I wasn't permitted by the landlord to have a space heater. I just opened my shades all day during daylight, and closed them at sunset. That's all, but that was enough to make my room noticeably warmer than ambient outdoor air temperature--so much so that my visitors from other units in the same building kept asking me if I was using a space heater on the sly. Try it; you might like it.