> As will now be shown, to perform an elementary logical operation in time ∆t requires an average amount of energy E ≥ πh̄/2∆t. As a consequence, a system with average energy E can perform a maximum of 2E/πh̄ logical operations per second. A one kilogram computer has average energy E = mc 2 = 8.9874 × 10¹⁶ joules. Accordingly, the ultimate laptop can perform a maximimum of 5.4258 × 10⁵⁰ operations per second.
https://arxiv.org/abs/quant-ph/9908043