I ask, not because I want to grow things that need to be much hotter in the winter time to grow (like peppers, tomatoes, etc), I know for those having supplemental heat would work great. I ask because I want to grow pineapples, which only fruit in the winter, and have to be around 65 degrees F to do so. Obviously we don't really get 65 degree winters, but I am trying to design a greenhouse system specifically for bromeliads, and this is a harder to find answer.
I have no greenhouses (and I'm mostly just posting this to give you a BUMP ), but I think the temperatures would vary greatly on how much mass it had to retain heat, and how much sun they get during the day. If it's in the shade most of the day, it will not be nearly as warm as if it were in full sun. Also, the amount of rain/clouds verses sun will also affect it greatly. Those cloudy days can be horrible for solarenergy. For instance, I live on a north-facing slope, and on a really rainy/cloudy day in the winter, my solarfence will barely stay charged during the daylight hours, and will go dry at night. On a sunny day, it will almost reach full charge and stay charged over night. This is really the reason I do not plan on making a greenhouse as--one a sunny winter day--I only get 2 hours of sunlight. That's not enough to grow anything, let alone keep it warm!
But, if you have good sun, and a good build, you could likely get it pretty warm in there. For instance, there were some people (http://www.permies.com/t/46532/greenhouses/Greenhouse-Future) who made a green house with a LOT of mass to retain the heat of the sun, and a clear roof angled just right to let the sun in to warm up the mass and grow the plants. According to their ebook, it was 10 degrees F outside, and 64 degrees inside. But, this was also New Mexico, where they obviously have much longer winter days, as well as sunnier winter days. Here's the link to their free greenhouse ebook (http://richsoil.com/greenhouse-ebook.php).