calculations for heating and venting the greenhouse

using some helpful internet calculators, i've figured out this much:

fan calculator:
9' length
5' width
7' height
W x L x (H x 1.2) = 378 CFM fan needed (cubic foot per minute of air moved)


heater calculator:
exposed Surface Area of greenhouse
north wall = 9' x 7' = 63
south wall = 9' x 6' = 54
west wall = 5' x 6' = 30 (estimated because the east and west walls are actually ...
east wall = 5' x 6' = 30 (a square with a triangle on top and i'm not doing that much math)
roof = 9' x 5' = 45
exposed Surface Area = 222
internet calculator said 226, so i'm going with that number.

SA x DR = BTU
DR = degree rise, minimum nighttime temp you want to keep inside the greenhouse minus the coldest outside temp you'll be heating against (in Fahrenheit). 55-15=40
226 x 40 = 9040 BTU heater needed

alternately, SA x DR x HL = BTU
HL = heat loss thru single pane 1/4" thick (6 mm) glass. the chart said .9
226 x 40 x .9 = 8136 BTU heater needed


heating cost calculator:
used SA, DR, HL, and # of heating months

cost per month based on needing 8136 BTU heater:
(based on the nighttime temp being constantly 15` outside and 55` inside the greenhouse)
927 KWH of Electricity (kilowatt hours)
35 gallons of propane (per freaking month, that is more than one gallon tank per night!)
3101 cu ft of natural gas

cost per year, estimating 4 months of heating required (guessing march, april, sept, october)
3706 KWH of electricity
138 gallons of propane
12405 cu ft of natural gas

927 KWH of electricity in one month is more than my entire march electric bill use was! (660, feb-700, jan-830 KWH).

i looked around online at some greenhouse heaters.

of the 120v variety (can plug in directly to any outlet), most only rated 1500 watts and 5120 BTU's, which is less than what i supposedly need. googling around to understand volts and watts and amps reveals that 120v on a 20 amp circuit gives 1500 watts. 120v on a 30 amp circuit gives 3600 watts. i dont know what the circuit for the garage is rated. i'll have to go downstairs and look at the circuit breakers. i was hoping to be able to just run an extension cord from an outlet in the garage to the greenhouse. i didnt see any plug in heaters rated for 3600 watts. these 1500 watt heaters tend to run $50-100.

of the 240v variety (needs to be wired to its own dedicated circuit and its own outlet, like a stove, dryer, or air conditioner require). they tend to be rated for 19000 BTU's. which is over twice what i need. aaaaand these heaters tend to run in the $200-500 range.