Mining hardware efficiency, and why the number moves

Efficiency is a ratio: the power a machine draws divided by the hash rate it produces. It is usually quoted in joules per terahash, which is the energy spent to perform one trillion hashes. A lower number is better, because it means the machine earns the same revenue for less energy. The figure is a specification of the machine, not of the network, and it is measured at a stated operating point — a machine run at a lower voltage may be more efficient per hash but produce fewer hashes overall.

The number matters because energy is the dominant operating cost of mining. A machine that is twice as efficient as another earns the same revenue for half the energy bill, which is the difference between profit and loss at a given tariff. This is why efficiency is the specification miners compare first, and why manufacturers compete on it generation after generation.

The figure is also the one that goes stale fastest. A machine that was competitive when purchased can become unprofitable without any change in the price of bitcoin, simply because more efficient machines have entered the network and pushed the difficulty up. The efficiency of the installed base is therefore not a fixed number but a moving frontier, and any comparison of machines must state the date it was made.