Jim, for my 12 x24 greenhouse, I can keep it 60 degrees with a oil filled radiator heater and a oscillating stand fan. The cost for me to run it on high for 24 hours would be $3.96
@ 11 cent per kwh. Instead of a tarp, I like to use a clear solar pool cover as an extra blanket when it gets real cold here. The kind with the bubbles works good for me.
Its heavy duty and uv resistant. I found mine on ebay, it was cheapest and had free delivery.
