I use an electric fan heater with a thermostat to keep my greenhouse frost free. I have also insulated the greenhouse with bubble wrap to help reduce heat loss. I haven't noticed a significant increase in my electricity bill.
I use a paraffin heater but I only use this to start my seeds of that would be in about 2 weeks time, It works for me as I don't have electricity in greenhouses and don't use it long.
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