the efficiency, in first, is HIGH dependly by the model ... so , 38th / 40th / 42th
for exemple, the 42th model have better efficiency than 38th model , the chips are selected, and the best is on the 42th model ...
And the efficiency is too depend on your environement, if you run at high temp place, efficiency down ... but in very cool temp, better efficiency,
i see on a year, on 5c ambient temp VS 40c ambient temp, the efficiency change to 10% ! so is important point...
At 50th, depend on your model, but for 42th model with arround 25c temp, give arround 2400w at wall ,
you need to know, the best efficiency is optained at downclock (is available on all models).
with pleasure
but is perfect working without any problem ... 
