The unit power of a mining machine is how much hashrate it can generate from 1 watt of power. It measures the machine’s efficiency. The less unit power the machine consumes, the more efficient it is. Unit power of Bitcoin machines is measured as W/T, referring to how many watts per THash/s. Another way to measure it is in Joules, as 1 Watt = 1 Joule per second. For example, the most efficient Bitcoin machines can reach less than 25 W/T or 25 J/T.
Check popular miners and their unit powers here.