A heated greenhouse provides much greater control over your growing temperature than a greenhouse that relies solely on passive heating does. However, you must consider the cost of heating the greenhouse before you install the heating system. The following procedure for estimating your heating costs requires you to have some basic arithmetic skills and knowledge about your greenhouse.
Measure the height and width of each wall and roof surface of your greenhouse. Multiply the height by the width of each surface to obtain the area for that surface. Add all of these surface areas together to get the total surface area of your greenhouse. Call this result value A.
Determine the temperature at which you wish to maintain the interior of your greenhouse. Call this temperature value B.
Determine the average low temperature in your area for each month of the year. For example, this information is available for Toronto at World Weather Online. Record the temperature for each month where the average low temperature for that month is less than value B.
Add these temperatures together and divide this total by the number of months in a year where the average low temperature is less than value B. Call this result value C.
Refer to the documentation for the glazing on your greenhouse to determine the R value for that material. Divide 1 by this R value to get value D.
Multiply values A, C, and D together to get the average number of British Thermal Units that your heating system needs to produce each hour. Multiply this result by 12 to get the average number of BTUs it needs to produce in a day, since your heater will typically be running only half the time.
Multiply this result by 30 to get the average number of BTUs your heating system must produce in a month. Call this result value E.
Obtain the number of BTUs that your heating system produces per unit of energy that it consumes. Call this value F.
Value F will be in units of BTUs per kilowatt if your heater is electric. This value will be in units of BTUs per gallon if your heater uses propane, and it will be in units of BTUs per cubic foot if your heater uses gas.
Divide value E by value F to determine the average number of energy units that you'll consume each month. Call this value G.
Obtain the cost of an energy unit from your utility company. Call this price value H. Multiply value G by value H to obtain the average monthly cost of heating your greenhouse.
Contact a heating contractor, such as Climec Residential, in your area to obtain a professional assessment of the heating requirements for your greenhouse. These contractors can also install and service your heating system.