It depends on how much heat you need. How cold does it get there and what temperature do you need to maintain. If you need very much supplemental heat at all you're not going to get by without running power. An extension cord can deliver 1000 W, which corresponds to 110 lbs of granite per hour, if the rocks are heated to 212 F and cooled to 50 F.

You want the good news or the bad news?

Trying to keep old man winter out of a GH even 6x6x6 is gonna cost ya.

But if you bury it (solar heated pit greenhouse) you might have a shot at it.

Just an outside opinion, YMMV..

So how did the greenhouse work out? In my climate, which is "colder" than yours, being in the sun from sunup to sundown even in the dead of winter would cause overheating many days and then freezing at night. Not many plants can take that, although there are a few. Small greenhouses take a lot of attention, venting when they get hot, heating when they get cold if the plants in them are not at least moderately hardy, watering if you live in an arid climate (like I do), venting moisture if you live in a wet climate.

And what flowers did your wife want to keep blooming all winter? Pansies and violets will bloom all winter outdoors for me with just a bit of protection (they are close to the house and mulched, but not covered). And of course flowering kale and flowering cabage look like great big roses in the garden much of the winter (and are even edible, in fact the deer just love them!). Houseplants like hoya (Grandma's Old Fashioned Wax Plant and Indian Rope Plant are two) and Christmas/Easter cactus (botanically known as Schlumbergera or Zygocactus) generally bloom in the winter with only moderate care, and don't need to be forced. OTOH, you can grow paperwhites, or force tulips or dafodils without much effort, and without a greenhouse.

Wow!....That's a really good idea but candle holders are also most useful for decorating your home.

Here is a link that might be useful: candle holders