While sifting through all the growth rate data I collected last fall, one thing I'm looking at is fuel use, and if using more fuel to speed up growth rate is cost effective.
The greenhouses are heated by a furnace that blows hot air out (left). They have a meter on them (right) that shows how many hours the blower has actually been blowing air out. I record the number twice a week. Since the meter is continuous I adjust the numbers to reflect the number of hours it was on for my particular experiment. Oh-so-conveniently, the furnace uses one gallon of fuel every hour, so I also convert "hours" to "gallons."
The above chart shows total fuel use in each greenhouse. Greenhouse one is heated to a 40 degree (F) minima and greenhouse two is heated to a 50 degree minima. This doesn't mean that each greenhouse is at a constant temperature, it means the temperature will not fall below the given set point. You can see this created a big difference in how much fuel was used.
Since 'gallons of fuel' is a bit arbitrary to the non-greenhouse owner, lets attach monetary values (everyone understands dollar signs). So,
Greenhouse 1: 108gal x 3$/gal = $324
Greenhouse 2: 264gal x 3$/gal = $792
* As usual, this data pertains only to our greenhouses this fall. It is not complete and should not be used elsewhere. If you are looking for data pertaining to winter greens growth, fuel use and temperature you'll just have to wait until I'm done with my thesis! *