  The academic literature contains many estimates of the resources required to
operate a cryptanalytically relevant quantum computer (CRQC) in terms of rather
abstract quantities like the number of qubits. But to our knowledge, there have
not been any estimates of these requirements in terms of more familiar economic
resources like money or electricity. We demonstrate that the electrical energy
required to break one cryptographic public key can be decomposed into the
product of two factors. There is an extensive literature of previous estimates
for one factor, the spacetime volume, that range over about six orders of
magnitude; we discuss some interesting patterns in these estimates. We could
not find any quantitative estimates at all for the other factor, the average
power consumption per qubit. By combining several data points from existing
superconducting-transmon quantum computers and extrapolating them to enormously
larger scales, we make an extremely rough estimate of a plausible value of
about six watts/qubit consumed by an eventual superconducting-transmon CRQC. By
combining this estimate with a plausible spacetime volume estimate of $5.9
\times 10^6$ qubit-days from the prior literature, we estimate that - even
after expending the enormous costs to build a CRQC - running it would require
about 125 MW of electrical power, and using it to break one public key would
cost about \$64,000 for electricity alone at current prices. Even if a CRQC is
eventually built, merely operating it would probably remain the domain of
nation-states and large organizations for a significant period of time.
