To figure your expected propane use, I used the following assumptions- for zone 6, a minimum outside temp. of -5 deg.
A 60 deg desired inside temp., and your structure is twin wall, 4 mm polycarbonate. Your surface area is 1600 square feet. The propane heater will run an average of 12 hours per night, delivering 72800 btu per hour. If any of these numbers are not correct, we can change them easily.

propane use per day = 9.48 gallons (0.79 gal./hr)
30 days per winter month use = 284.4 gallons propane per mo.
The propane delivered to me yesterday in California was $2.40 per gallon. I rent a 600 gallon tank for $75 per year.

Propane costs differ greatly depending on where you live, but you are correct, it will cost you a fortune to keep your greenhouse at between 50-60F in your zone.

My greenhouse is 36 x 48 (1728 sq ft) singlewall Lexan and I have a 200,000 btu heater. I am in zone 8 3/4-zone 9. It only gets below 32 here maybe 3-4 nights a year, sometimes for only a few hours a night. It certainly CAN get into the teens and historically has done so, but not since about 1989. I don't have twinwall because, here, its much harder to cool the greenhouse in summer when its 85-95F than it is to heat it when its only below freezing for a couple of nights a winter.

My propane is $1.90/gallon and my tank rental is $40/year.

I have run this greenhouse for 4 years now keeping the nighttime winter temp at at least 55F, and my average yearly propane cost is between $1500- $2000/year.

I grow ONLY tropicals in my greenhouse, many of them somewhat rare. I have some palms that will die at temperatures below 40-45F so its necessary for me to keep my greenhouse at least 55F to be certain that things thrive and don't stall. If you have plants (even if they ARE tropicals) that can tolerate lower overall temps (say, 40-45F) for the winter, you can save yourself a lot of money by turning down your heat. Check my tradelist and see what I have that requires the 55F heat.

If you are growing stuff like brugmansias, colocasias/alocasias, fruit trees like citrus, cannas, plumerias...you can get by with less heat.

If you are growing orchids, heliconias, very tropical gingers, and some of the more rare palm species etc you may need the 55-60 heat.

Exactly what tropicals are you growing?

>I realize this will probably cost me a fortune!

That's good cuz you're likely correct about this! Susan you've not said what heater(s) you have installed. You will need to do some serious ciphering in re: the heaters' BTU/hour and the expected burntimes etc. The BTU per gallon for propane is 91,502.

I think you would do best to inquire of some local nurseries to see what they do and what tanks they have connected, also what contractors they could recommend for installing a heater/tank system for you.

Here is a link that might be useful: facts about propane

Not all my plants are tropicals, but I would like to keep it warm enough for them. I guess I'll try to partition off sections to keep them varying temps, and try the water barrel method.
Some of what I grow: Aroids, Clivia, Succulents, Eucalyptus, Chorisa Speciosa, Sequoias, and some other quite rare oddballs. I'd like to try growing some citrus and other unusual trees (keep them small by growing in pots).
I don't have enough light or space in my house to grow indoors. In the past I've been able to keep my plants in the house or basement under lights but that isn't possible here.
We recently moved to a rural area, and I didn't realize just how rural it is with nearest retail nursery about 50 miles away and contractors that just scratch their heads when looking at greenhouse directions. They are willing to be paid $30/hr each, for a crew of 4, to stand there and read the directions and try to figure them out- NOT!
Bihai, you live in zone 9 and pay that much? Yikes! I'm really in trouble!
Susan

Before you build really think about all this in your zone. I have a greenhouse that is 18x25x (9 ceiling). I use a 30,000Btu gas heater and I keep my temps in the low to mid 50's, around 52 to 55 or a take higher sometimes. On night that it will not go below 40°F, I will cut off the heater. I watch and I can run less than $90 a month. This winter has been warmer than last, so Feb will be the only month in which I will pay $90. Dec, Jan, March were and will be warmer than normal months.

So, look at your size and make sure you really need the space, make sure you can fill that greenhouse up. I am almost a zone 8, so my cost is not much at all.

You can see my greenhouse if you look down a few thread, on the main Greenhouse page. I have my place loaded with stuff.

I am hoping next month I can start working some of the stuff out. Can you believe spring is just around the corner?

Oh, I'll have no problem at all filling it. As we are all rock and clay here almost everything will have to be potted. I have probably 5,000 Calla Lily bulbs that I need to pot up or get rid of. All my lilies, hardy glads, and other flowers will have to be in the greenhouse. For the last 11 years I've lugged pots out every spring and in every fall. No more! (OK, maybe just a few) And I've never known anyone that hasn't been able to fill their greenhouse, have you?

I think having 8mm PC will help keep the heat in, and by the time the GH arrives we'll be into March so won't need too much heating. For next winter I'm sure I'll be sectioning off parts of the greenhouse, but I really need to get my plants into the light. We get maybe 2-3 hours of light actually shining into the house a day, due to porch roofs cutting off light. It's like living in a cave.

We're also thinking about some sort of wood stove for the daytime hours, lots of fallen limbs and driftwood around here.

Yes, I pay that much in zone 9a. But, in addition to having 1728 sq feet, I also have a 20 ft ceiling. Most home greenhouses have a ceiling height of 8-10 feet. I have twice that in upward area. Heat rises, so I have fans in the attic that circulate the heat and blow some of it back downwards. But my heat sensor is located at about 6 ft, to detect the temps on the floor and turn the heat on and off at the level where plants actually grow.

For me, a temp of 40 in the greenhouse would be bad. SOme of the stuff I have needs closer to 55. My rule of thumb for choosing plants for the greenhouse is, if it will grow outside here, it doesn't get the greenhouse. And, if its small enough to stay potted so that it can be taken inside for a freeze, no greenhouse.

Its certainly true that were it not for some specific plants I baby, I could probably turn down the heat to the 45-50 range. But if I did, some of the more marginal highly tropical plants I have might not die, but they would stall out or go dormant. Since that's not my aim, I keep it warm. I had my propane tank refilled for the first time in September (to get ready) and it has been such a warm winter that until late January I had hardly used any propane at all this year. But although to date we have only had one night that it actually froze enough to damage anything outdoors, we have had many nights between 36/37-49/50. So I have used propane even in this warm winter.

I also make it a point to "seal" the greenhouse in winter. There are a few places that let in draughts, so I take bubble wrap and plastic sheeting and tape the leaky places closed, which helps me retain some heat. I know that there is a lot of stuff you can do with a smaller greenhouse to save heat, like line the walls with clear insulation etc. That would be too much for me, given the wall area I have. Just do a lot of research and see what you can do to minimize your propane usage to an affordable range for you.

Honeybunny, if I were you I'd get the biggest tank I could afford. They only put 400 gallons in a 500 gallon tank (80% is full). Cuestaroble's numbers sound a little low to me cpmpared to my past propane usage. I have a 20x23 glass GH which I have in the past kept at 54F...this year I ticked it up to 57F. I run it off of the tank to the house, 1500 gallons. We are on a monthly fill program, so our tank really never gets under about 50% full.

Thanks so much to everyone for their comments. I think I will try to get a big tank. Stay tuned for construction photos!
I'll try to edit out any violence between my DH and myself- HA