A hobby greenhouse can significantly extend the growing season of your frost-tender plants. It allows you to start your plants several weeks before the average last frost and grow them for several weeks after the average first frost in the fall. Place your greenhouse in full sun and the temperature inside will soar 30 degrees or more over the outside temperature during the day. However, since a hobby greenhouse has very little insulation, the temperature inside will quickly plummet to the outside temperature once the sun goes down. Because of this, you will need to provide supplemental heating on nights the temperature falls below freezing to ensure your plants do not perish.
Calculate the square footage of each outside wall and roof in your hobby greenhouse. To calculate the square footage, measure the length and width of each wall or roof and multiple those two figures together. Add all the square footage together to find the total square footage of all exterior surfaces.
Use a greenhouse heat calculator to calculate the BTU (British Thermal Units) needed to heat your greenhouse. These calculators will compute the needed BTU after you enter the square footage of the exterior surfaces, the minimum outside temperature you expect to experience, the temperature you want to keep your greenhouse at and the heat loss value of your greenhouse. See Resources for a good calculator that includes heat loss values for common greenhouse materials.
Convert the BTU needed to watts by dividing the BTUs by 3.14. Electric heaters are sized using watts. By knowing how many watts of heat you need, you can purchase the correct size heater.
Set up your electric heater in your greenhouse and plug it in. Set the thermostat on the electric heater to the temperature you desire to keep your greenhouse.
Place a thermometer in your greenhouse in a location that is out of direct line with the heater. This will enable you to monitor the temperature inside your greenhouse to see if the electric heater is keeping the temperature sufficient.
Turn your heater on and check the thermometer occasionally to ensure the greenhouse is staying warm on those frosty nights.