Greenhouses can greatly extend your growing season in northern climates. They also allow you to grow more tropical plants that you otherwise would be able to grow in your climate zone. As autumn deepens you will need to provide more and more supplemental heat into your greenhouse at night, and eventually during the day in the dead of winter. The heating requirements vary depending on your greenhouse's size, insulation value and outside temperature. To calculate greenhouse heating requirements, you'll need to know a few details about your greenhouse.
Calculate the surface area of your greenhouse. The surface area is the total square footage of every wall and ceiling in your greenhouse. Square footage is calculated by multiplying the length by the width. See "References" for an easy-to-use greenhouse surface area calculation tool.
Use a greenhouse heater size calculator to determine the minimum BTU needed to heat the greenhouse. To use this calculator, you'll input the total surface area that you calculated in step 1, the minimum outside temperature that your area reaches in the winter, the inside temperature you want your greenhouse to maintain in the winter, and the heat loss value of your greenhouse. See "References" for a convenient calculator. The calculator also lists the heat loss values of common greenhouse materials.
Convert the BTU total to watts if you are going to use an electric heater to heat your greenhouse. One watt equals 3.413 BTU. (e.g. if your total was calculated to 10,000 BTU, you would need an electric heater of approximately 2,929 watts).