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How does it know power consumption?

It's based of that 40 megahashes per second consumes 1 joule of power (1watt)

Where do you get that figure? ASIC and GPU devices are completely different in power consumption for the same amount of MH, for example.

ASICs represent only a tiny fraction (for now) of the power consumed by the network though. The vast majority is consumed by GPUs at ~1-2 MHs/watt.