Answer: Based upon your description I'm going to assign a value to the entire inside area of your structure of 359 feet. I came to that conclusion using 8x10x8'. If your greenhouse is taller, you may have to reconfigure. Then, taking into consideration the lowest low temperature you'd receive in an average year (20F) and the temperature you'll want to keep the greenhouse (probably about 40F), and guessing that you have twinwall polycarbonate rather than a single fiberglass wall, I think you'll need about 5200 BTU's. You can purchase a 120v thermostatically controlled heater for about $60. Dayton is one manufacturer, Modine is another, but there are probably others. Try a websearch with the term "electric greenhouse heaters" and maybe you'll find others. Or, you might try contacting Steuber Distributing Co;
3rd & Pine St, Snohomish, WA 98290; Phone: (360) 568-2626. They have a catalog of nursery and greenhouse supplies and they sell both retail and wholesale.
Hope this information is helpful!
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