hairmetal:

What crops are you hoping to grow and how big of a greenhouse are you considering? This has a big effect on your management and design. Some crops need winter chilling and some don't.

I never heat during the day. I just keep it above freezing at night, 34F, or warm to 37F to maximize chilling. During chilling I keep it as cool as possible during the day. That usually works out to a high in the 50s. After chilling the high temperature, actually the temperature differental day/night, can have a big effect on blooming success of stone fruit.

If I knew your crops I might be more help.

About 20 X 20' if I do it right.

Mostly for citrus, cool season veggies, and some spring seed starting. Subtropical mostly, nothing that needs chilling. If I decide to try bananas I will probably have to err on the warmer side.

Here's another way to ask:

If I set my thermostat to, say, 47F, as an example...how much solar heating could I expect?

On a sunny day in December, I realize that it could top 80 or 90 easily even if it's in the 20s or 30s outside...but what about on a dreary, cloudy day, or a snowy day? How much solar heat inside the GH could I reasonably expect from the 47 baseline?

Although this area has more winter sunshine than my native Ohio, it still can be quite dreary.

hairmetal:

You are right that on a sunny day it can warm to 50-60F above outside. On a heavy overcast it might only warm 10F but even a moderate overcast can allow a nice warmup.

My citrus does very well, potted or inground, with 30s at night and 50s by day. This is the same temperatures as the main citrus growing areas in CA. They won't grow all winter but the fruit ripens and tastes very good.

So in zone 7 MD at least for citrus you won't have to heat during the day just keep it from freezing. If you are growing citrus as an ornamental, as many do, then you will need warmer to get that pretty growth all winter.

If you decide to grow something that needs chilling you could grow it in a pot and get chilling outside. Citrus can take a lot of heat so take that into consideration on your greenhouse design. Potted citrus can go outside during the frost free season. Potting makes many senerios possible.

FYI - i regularly overwinter my citrus in my unheated garage with no issues. Just remember to keep watering!

I did try to overwinter one in an unheated greenhouse and it survived through december but died back when outside temps dropped below 20F. The amazing thing was, even though outside temps reached -14F, the tree survived and sent new shoots out from the roots. Of course, that ain't good for fruit!! :-)

The question then is if I want actual sweet, edible fruit, is it enough to have my citrus trees "survive" or do I have to do better than that? Since the fruit actually ripens IN WINTER, esp. for mandarins and most oranges.