The solar heater will work a little on slightly overcast days, but for the most part it needs full sun to work. In the winter, this only accounts for about 30% of the days where I'm at. I do a lot of alternative energy projects from windmills, to solar air heaters to solar water heaters to solar ovens and even making ethanol at home. I also grow my own corn and burn it all winter to heat my house. Having livestock is new to me though, and for me is the last piece of the puzzle to becoming more independent. My end goal is to be as close to self-sufficient as possible within the next 2 years. I like the thought of not having to worry about how I will provide for my family during an extended power blackout, natural disaster, loss of income or an economic collapse. On the brighter side, if none of these desasterous conditions ever happen, then I'm at least saving money, helping the environment, keeping my family healthy and staying entertained all at the same time!
My next project is to make a small wind turbine for this chicken tractor that will put its power through a resistive heating element and heat the coop on windy days. Between the wind and solar heat, it should greatly help keep the temperature more tolerable in the winter! I may even put some rigid board insulation on the inside of the coop to hold the heat in better (I wasn't sure if the chickens would peck at it or not). I already have the scraps of insulation, just haven't put any in at this point.
Another future plan is to use install a "Power Wheels" battery and charge it using solar/wind, and in the winter have it run lights in the roost area to encourage better egg laying in the winter months. I'll be sure to return to this thread with pictures as I complete each of these projects.
PS - a note on these types of solar heaters.. They are very easy and cheap to make even for your home. A single heater the size of a single pane of glass from a sliding glass door can be made from less than $100 in materials will pump over 2000 watts of heat into your house on any sunny day even on the coldest of winter days. Anyone wanting more information on these types of solar projects (either for your coop, garage or home) should go to
www.builditsolar.com - they have hundreds of working examples!
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm shows a lot of excellent solar air heaters.
Here is a link to a guy that built a pretty big one for his home and it provides 100% of his home heat on sunny days:
http://solarairheater.lampresource.com/
I get free used sliding glass doors all the time and just remove the large tempered glass panels from them (you have to have tempered glass, as these solar heaters can and will get hot enough on the inside to crack normal glass). To date I have gotten 18 free panes of glass this way!