BTU calculator. Adjust the materials and shape/size of the various walls and such..
You can plug in various numbers such as your coldest winter temps to find out how much heat it takes to maintain your desired temperature on the worst case. Or you can use your Winter average temperatue for a few month period to find out what it will take on an hourly basis to heat over that time frame.
http://www.igcusa.com/greenhouse-btu-calculator.html
---------- Post added at 03:43 PM ---------- Previous post was at 03:06 PM ----------
I ran some numbers for fun.
Assuming 70 temperature average inside temperature... figuring your nights will be a little cooler and your days a little warmer. Not sure if you plan to just run a single temperature or moderate between day/night.
For outside average temperature I split the average low and average high for the end of December in your neck of the woods. That gave me a figure of 30degrees
6.25' wide, 8.4' long, side walls 6.4 feet high
8mm double wall poly carbonate (these kits usually have 4-6 mm poly carb. but I couldn't tell from the photo). I figured it was safe to use the 8mm figure though since it probably about balances between thinner poly carb and adding some bubble wrap.
plugging this in gives me a figure of 7000 btu/hour
So basically what this means is it will cost an average of 7000btu/hour to heat your greenhouse over most of Dec. Jan. and Feb.
So if you want to figure your cost.. 90 days at 7,000 btu/hour x 24 = 168,000 btu/day
Propane has 92,000 btu/gallon so it would take about 2gallons of propane/day. Propane is about $2.50 gallon.. so $5 day.
If you are planning to use electric then you probably need a 240v line..
A 240v 4000w heater will put out about 13,500 btu/hour
To figure how much electricity it would use..
7000 btu/hour x 24 = 168000 btu/day divided by 13,500 = 12.4 hours/day the heater will run x 4kwh (4000/1000) = 50kwh/day
Take the 50 and multiply by your kwh delivery charge and supply cost to find out the electricty cost/day
My power company charges 8.5 cents for delivery and 8 cents per kwh so 50 x 16.5cents = $8.25 day in electricty cost.
Plugging in the extreame temps during the night.. and maintaining 60degrees during a -15degree night. It will take 13,000 btu/hour to maintain 60degrees. So a 15-20,000 is should be sufficient.