Determining solar power system needs for 240 sq ft greenhouse

Hi all!

I have a 240 sq ft greenhouse (12x20) that I’d like to power off grid. I’ll have an exhaust fan, grow lights, misting/watering system, thermostat, and, ideally a small electric heater (though looking at other options) for winter. I’m trying to determine what system components I need for these energy demands. I live in Johnson City TN, where we get about 201 days of sun per year. In Johnson City, the summers are long, warm, and humid; the winters are short, very cold, and wet; and it is partly cloudy year round. Over the course of the year, the temperature typically varies from 28°F to 84°F and is rarely below 14°F or above 90°F. I will integrate other heating options (animals, thermal mass etc) so the heater is optional, though preferred.

Any help calculating/designing a system would be greatly appreciated.

Sarah Beth Hopton wrote:Hi all!

I have a 240 sq ft greenhouse (12x20) that I’d like to power off grid. I’ll have an exhaust fan, grow lights, misting/watering system, thermostat, and, ideally a small electric heater (though looking at other options) for winter. I’m trying to determine what system components I need for these energy demands. I live in Johnson City TN, where we get about 201 days of sun per year. In Johnson City, the summers are long, warm, and humid; the winters are short, very cold, and wet; and it is partly cloudy year round. Over the course of the year, the temperature typically varies from 28°F to 84°F and is rarely below 14°F or above 90°F. I will integrate other heating options (animals, thermal mass etc) so the heater is optional, though preferred.

Any help calculating/designing a system would be greatly appreciated.

The electrical loads will be impossible to properly calculate until you get the system components to verify their consumption. Is the grid available for the greenhouse? Your best bet if it is is to do a net metered system to bank kW hours in the summer to use them in the winter. If that is not an option then you would probably want to look into some seasonal growing and year long harvesting with grow light being an absolute last priority used sparingly. Electric heat on solar usually doesn't do well due to the reduced sun hours in the winter corresponding to the time of greatest need ...
So that was a whole lot of negativity! Solar is totally doable for the essentials it just has trouble with the high intensity tasks like heat and out of season lighting.
Cheers, David Baillie

luckily people have put in the legwork for this sort of task. Very handy spread sheet

Check this out. https://diysolarforum.com/resources/system-energy-audit-and-sizing-spread-sheet.12/

I used it to determine what my system would use.


I do think electric heat will be difficult to maintain in the winter.