Being on full throttle is not important when it comes to harvesting the energy from the MGUH. It's all dependent on the mapping of the engine and turbine design.
I've stated somewhere in this thread how much power can be delivered to energy store from MGUH, even posting a graph.
It's not far from possible for Mercedes PU to be constantly deploying MGUK because energy is avaialble from the H and battery.
The "self sustaining" power is what denotes the power that can be delivered to the MGUK from MGUH consistently over a the race distance. And can be considered the base power of the PU. No stored energy has to be requested for this base power level.
as to what amount of power, taking in to account energy transfer losses etc. and less than full throttle running, I am not sure how much on average is sent to MGUK on any given track. But I know it's possible for Merc to provide power to MGUK independent from the Energy Store.