How much heat/btu does your
greenhouse need per day? E.g 48,000WHr
How much heat does your solar panel produce per day? E.g 500W x 4Hr = 2,000WHr
With the above example the solar panel will provide less than 5% of the
energy needed.
Is it worth all the effort/resources for that limited gain. Could you maybe exhaust your furnace/dryer into the 'attached
greenhouse'. Spend the money on insulating the north-wall.
Now lets say the number add up and it is the best use of your resources.
I recommend using silicone insulated wires. Silicone insulation has good oil and solvent resistance, and good corona, and ozone and weathering resistance, has a higher temperature rater(more amperage), and a low halogen-toxin content.
I recommend adding a single super cheap battery and a low voltage cutoff switch for the fan/cable-heater, each for about $30. We don't care about the storage capacity(amp-hours) of the battery so we are not limited by the amount of cycles. And because the battery is in the
greenhouse we are not worried about efficiency. Without the battery we dont get max powerpoint and solar panels don't deliver their max energy. We would also have to setup a complicated logic board for 9AM when the solar panels are only ouputting 20W vs at 12-noon when they outputting 250W each.