Andreas Oehler at Fahrradzukunft made some efficiency measurements of some well run in IGH’s (Rohloff Speedhub 14 ca 5,000km, Alfine 11 ca 3,000km), at their different gear ratios compared to single speed and derailleur gearing: http://fahrradzukunft.de/16/wirkungsgradmessungen-an-nabenschaltungen
In part 2 he measured and compared the Rohloff Speedhub 14, Alfine 11, Inter 8 and NuVinci N360: http://fahrradzukunft.de/17/wirkungsgradmessungen-an-nabenschaltungen-2
In part 3 he measured and compared the Pinion 1.18 to Rohloff Speedhub 14, Alfine 11, and Inter 8: http://fahrradzukunft.de/20/wirkungsgradmessungen-an-nabenschaltungen-3
Overall lesser efficiency at 50W is likely due to proportional higher losses from seals, chain tensioner, gears, ect (eg if you have overall gear train losses of 10W, you got proportional higher overall losses at 50W power input than at 200W power input)
The articles are in German, but Google Translate will make them readable in Englisch. Take some time to understand the grafic’s. They might look a bit confusing at first.
The graphics show, the Rohloff isn’t just the most expensive IGH with the highest gear count (exept the bottom bracket mounted Pinion 1.18), it got the smallest efficiency losses, regardless of power transmission, as well.
You might take these measurements with a grain of salt. Tester Andreas Oehler himself mentioned at the end of part 1 some potential measurement errors. Regardless, I find these comparisons highly interesting, even if the actual measurements aren’t 100% correct. Even if it where, his clean chain drive might have diffent losses than my dirty chain drive. In my view it is enough to see a more or less correct tendency in efficiency differences between the different drive train variants.