Answer: For greenhouses, the answer to your question depends on a number of variables including how the structure is built, the crop/heating requirements and the local cost and availability of various power sources as well as the installation and purchase of the system itself. There is really no one single best answer beyond "Whatever is relatively cheap and doable." Often your local power company can help you figure out the heating needs and sample computations for relative costs to help you make the best decision.
For cold frames, the answer it simpler. These are not usually heated at all, although occasionally a gardener will run a heating cable or possibly a light bulb to take the edge off particularly cold nights.
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