To be fair, the furnace could in principle use some form of heat pump to achieve the necessary temperatures, while using energy from a 500°C source.
That said, it would likely need to use a little electrical power to run a compressor and control systems (though in theory, it's possible to build purely heat-powered heat pumps, just mildly impractical) ... and current “super-high-temperature” heat pumps only go over 100°C, but that's clearly not a problem for in the Factorio sci-fi future.
Silly-seriousness aside, thanks a lot for the mod, I'm pretty hyped to try it in my current playthrough once I get nuclear power going (I'm trying the Nuclear Overhaul mod) ... and I might just skip electrical furnaces entirely, since I wasn't planning on deploying them before getting powered by spicy rocks.
