Is this even possible? Greenhouse/living space question.?

I found a few links online about how to build a greenhouse that will maintain enough heat on a sunny day (even in Winter) so that one can include a loft and sleep out there comfortably. Do you really think that's possible with no electricity? I read about ways to maintain the heat (like by constructing the windows at the right angle to maximize the sun potential, putting special materials beneath the flooring, and putting lots of sand- filled milk jugs painted black around the outside, etc. but it still seems unlikely that it could work in a New England winter. The articles even went so far as to say that the structure should stay warm (enough) even if there is minimal sun for a couple days after a sunny day or two. What do you think? Anyone seen this work?