Our sitting room has a large glass front facing North (note to my Northern hemisphere readers: this is the sunny side!), and I use this glass front to warm the house during daytime in winter. The trick is to know when to open the curtains. Open them too early, and the single pane windows will lose all the warm air. Leave it too late, and I have missed some of the beneficial warmth of the sun.
That is where my energy efficient, friendly and totally natural cat-o-meter (seen in action on the left above; and on the right after he has done his job) comes in. My cat seeks out the warmth and will crawl behind the curtains when the temperature is just right. I then know it is time to let the light in and open the curtains. On cloudy days, the cat simply stays on the sofa.