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If you live in a cool climate, you should consider purchasing a greenhouse heater. The amount of heat generated by a heater is in BTU's (British Thermal Units). In order to detemrine how many BTU's you will need for your greenhouse, you will need to gather some information about your greenhouse.

First you need to calculate the total square footage of the exposed surface of your greenhouse. This does not include the floor.

You then need to determine the maximum inside temperature you want in the your greenhouse. Since you are growing seedlings, you should keep the inside temperature at 60 degrees.

You then need to determine the minimum outside temperature you expect in your area. Since our winters can be unpredictable, make this temperature lower than your last winter.

You now need to determine the heat loss factor for the greenhouse covering material. Twin-walled polycarbonate (3.5mm and 5mm) has a heat loss value of .8; 8mm polycarbonate has a heat loss value of .62.

You now can calculate the required BTU's: Multiply the surface square footage x the difference between your desired inside temeprature minus the coldest outside temperature (e.g. 60 degrees minus 20 degrees = 40 degrees) and then multiply this by the heat loss factor.  For example, assume your square footage is 288; your inside temperture is to be 60 degrees and your coldest outside temperature is 20 degree; and your heat loss vaule is .8:

288 (sq. ft.) x 40 x .8 = 9216 BTU's is the minimum BTU rating needed to keep your greenhouse at your desired temperature during the coldest months.