I've done a walk through of various greenhouse/solar combinations in a few other places here.
R: If you want to keep the fish happy, don't do it in a green house.
Net annual heat requirement for a greenhouse
= 24 * Footprint * DD /2R with the answer in BTU.
So your typical 2 ply plastic hoop house 30 x67 feet So Footprint = 2000 square feet.
DD = degree days of heating. This is normally calculated against a 70 F reference. So if the temp is 69 for 1 day, that's a degree heating day. If it's -30 for 2 days that's 200 degree heating days.
R is the average R value of your greenhouse. For plastic, it runs about R1 per layer of plastic.
The 2 comes from an approximation that for every square foot of floor there is 2 square feet of roof or wall. Pit houses, and chinese style ones get away from that.
The 24 comes from the conversion of days from the degree days to BTU/hour that R values are measured in.
A square foot of double plastic green house in my 10,000 degree day heating climate takes 240,000 BTU/year
1 BTU changes 1 pound of water 1 degree F.
So if we limit our temperature chnge to 20 F it takes 12,000 pounds (240,000/20) of water to store a year's worth of heat.
At 60 lbs/cubic foot (yeah, it's bigger than that, 62.5. Cut me some slack) that's 200 cubic feet of water.
You aren't going to store the whole year's heat.
Let's figure that even on a cloudy day we're getting close to enough sunshine, so we're really only heating it at night.
That cuts the requirement in half. 100 cubic feet.
Let's do a higher efficiency roof -- R 4, isntead of R2. 50 cubic feet.
Let's seperate the greenhouse from the water storage, and try to store water at a differential of 100 degrees instead of 20 degrees. 10 cubic feet.
Let's use this hotwter tank building as the north wall of our greenhouse, and change that 2 factor in a 1.4 factor. 6 cubic feet.
So now for each square foot of green house we want a squarefoot of hot room filled with barrels of water.
That is do-able. Google "Nick Pine" solar heat storage.
Is it the best answer?
Here's a better one:
Make a high effiency rocket stove type heater and use it to heat a surplus service station gasoline tank full of water. They are typically 20,000 gallons = 160,000 pounds of water. Raise the water temp 100 degrees you have 16,000,000 btu.
On a -30 day, you need 2400 BTU/square foot so your 2000 square foot green house needs 4,800,000 So you have to fire up the stove once every 3 days. If the temp is only down to 20F you fire it up once a week.
How much wood? Various charts put wood at 20 million to 30 million per cord of wood. So you'd go through a fair bit of firewood.
A million BTU is about a gigajoule or 10 therms. Right now I'm paying 3 bucks a gigajoule for natural gas. So those bitter cold nights would cost me about 15 bucks a night. Or if I heated strictly with natural gas about 1500 bucks a year.
Sure off grid is worth it?
Maybe a trout pond would be easier.