The SP20 can be under-clocked to ~0.5 J/GH, depending on cooling and specific ASIC batch.
Cooling?
Ambient temperature.
any specifics? it has to be hot (>25C), cold (<18C) or in a specific range?
Measurements were done with Emerson 1600.
It offers (according to spec) minimal efficiency of 92% in our tested working points @25C ambient.