Greenhouse Energy Calculations
A cost comparison of different energy sources for heating a greenhouse
In order to determine the heating costs for a typical heating season from September to May, unit cost for each fuel source must be determined as well as the total energy required to heat the greenhouse.
Example of a calculation:
Greenhouse floor area = 496 m^{2}
Typical energy required by a double poly greenhouse = 2100 MJ/m^{2} (2.1 GJ/m^{2})
Unit Costs  Energy Content 

Natural Gas = 25.53¢/m^{3} Propane (liquid) = 40¢/1 Fuel Oil = 39¢/1 Electricity = 6¢/kwh 
0.0372 GJ/m0.0372 GJ/m^{3}
0.0269 GJ/1
0.0390 GJ/1
0.0336 GJ/kwh 
Note: Please note that the above unit costs were accurate at the time of publication. Variability in the price of heating fuels negates any attempt to produce standard unit costs. In order to calculate your heating cost please substitute the current unit cost for the appropriate fuel source.
The combustion efficiency of these fuels is typically 6080% of the above values of energy content except for electricity, which is usually considered 100% since there is no combustion. Even with high efficiency combustion furnaces, actual efficiency of heat transfer is only 70% on average. Therefore to determine the energy cost per unit of usable energy ($/GJ) and the cost per square metre, the following formulas are used:
Cost ($) of fuel/litre Energy content of fuel (GJ/1) 
Will yield: Cost ($)/Giga Joule [unit of energy].
Then considering 70% efficiency we divide this number by 0.70.
$/GJ 

Energy cost at 70% efficiency
Then multiply by 2.1 GJ/m^{2} (typical energy consumption of a double poly greenhouse).
Will yield: Cost ($)/m^{2} (floor area).
Then multiply by greenhouse area (for example, 496 m^{2}).
Natural Gas
 Unit cost  class 4 commercial
 $0.2553/m^{3}
 0.0372 GJ/m^{3}
Then  $0.2553/m^{3}0.0372 GJ/m^{3}  = $6.86/GJ at 100% efficiency= $6.86/GJ at 100% efficiency 
$6.86 GJ 
= $9.80/GJ at 70% efficiency= $9.80/GJ at 70% efficiency  
$9.80/GJ x 2.1 GJ/m^{2} = $20.58/m^{2} floor area  
$20.58xm^{2 }= $10,207.68/season 
(Note: approximation based on average weather conditions)
Propane
 $0.40/1
 $0.0269 GJ/1
$0.40/1 
= $14.87/GJ 
$14.87/GJ 
= $21.24/GJ 
$21.24/GJ x 2.1 GJ/m^{2} = $44.61/m^{2}  
44.61/m^{2} x 496 m^{2} = $22,126.56/season 
Fuel Oil
 $0.39/1
 $0.0390 GJ/1
$0.39/1 
= $10.00/GJ 
$10.00/GJ 
= $14.28/GJ 
$14.28/GJ x 2.1 GJ/m^{2} = $29.99/m^{2}  
$29.99/m^{2} x 496 m^{2} = $14,880.00/season 
Electricity
 $0.06/kwh
 $0.0036 GJkwh1
$0.06/kwh1 
= $16.67/GJ 
(note: 100% efficiency)  
$16.67/GJ x 2.1 GJ/m^{2} = $34.99/m^{2}  
$34.99/m^{2} x 496 m^{2} = $17,355.04/season 
As with any cost calculation, individual circumstances will need to be taken into account. For instance the use of wood and other combustible plant material (straw, coal, wood or aggregate pellets) in most instances will accommodate the needs of the producer provided the proper equipment is implemented and the source of fuel is reasonably close given the bulky nature of these materials. Handling and specialized equipment expenses will also need to be taken into account when deriving a cost calculation. Typically these "alternative" fuels require constant supervision as delivery and consistency of energy content can be quite variable. For this reason even though the basic cost per unit of energy is lower other considerations will need to be factored into the equation when comparing heating fuel sources.