Several comments have hinted at the possibility that this would be a over-unity device but as I couldn't find a specific demonstration, here is one that does no

Several comments have hinted at the possibility that this would be a over-unity device but as I couldn't find a specific demonstration, here is one that does not involve any fancy physics. Let's assume we have a spacecraft powered by an energy source with total energy `E` and mass `m`. The EM-drive produces a constant thrust `F` at a constant power `P`. Kinetic energy of the S/C after time `t`: Ek = 0.5 * F^2 * t^2 / m If `t` is the time at which the energy in the source is exhausted, then the ratio of kinetic energy developed to the expended energy (E = t * P) is: Ek/E = 0.5 * F^2 / P^2 * (E/m) So this basically says that we have a over-unity device whenever the energy source has a specific energy above: E/m > 2P/F ~ 1.6 10^6 J/kg which is certainly not achievable in practice, but in theory is totally fine being still well below c^2 (which by the way justifies a-posteriori the neglect of relativistic corrections in the above).