Last winter my boys and I made a very cheap solar heater with a large south facing window in our second story computer/ homeschool room. We just hung a black plastic sheet on the inside of the window with curtain rods. This left about 1.5″ for natural convection to take place. It draws the cooler air near the floor and the heat just flows out the top. This room would always get warm with the sun shining in, but with just this black sheet I was able to turn off my heater (forced air) and just leave the fan on to circulate this heat throughout the house. I also used a portable fan to move heat out of that room so we could stand working in there on our computers. On sunny days with below freezing temperatures outside , I could shut off our heater from 11:00 am to about 6:00 pm and stay perfectly warm and comfortable inside the house.