I noticed that the furnaces (excluding electric), have an energy consumption.
They stop working automatically once the coal runs dry, not when the power goes out, if you manually insert/remove it in.
So, realistically they should not affect the energy consumption.
There are different types of fuel. https://forums.factorio.com/wiki/index.php?title=Fuel
You can use to warm up water to generate electricity or - like anything with a burner (furnaces, car, train) - use the containing energy directly.
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