A lot depends on what you want to grow.
If you were just wanting to keep the frost off a small paraffin cold frame heater would do. I have one which I use in a partitioned section within my greenhouse just for frost protection. It is a parasene model (no idea of number). You can supposedly use it for 14 days without a refill but I tend to get a week from a filling.

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My Majesty made for him a garden anew in order
to present to him vegetables and all beautiful flowers.- Offerings of Thutmose III to Amon-Ra (1500 BCE)