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I thought it was hotly contested and I've seen figures suggesting the linear relationship of input to output is less steep than with a chain such that they become more efficient than a roller chain at around 210W.
Others scream about 10% losses vs 5 for a well lubed chain so who knows
PhilDAS-
the linear relationship of input to output is less steep than with a chain such that they become more efficient than a roller chain at around 210W.
I imagine you got your "around 210W" figure from the Friction Facts paper, where it is suggested that a hypothetical belt drive requiring no preload would be more efficient than a chain above 208W. They state that since no such systems exists at present, the chain will always be more efficient than the belt because preload needs to be increased to correspond to higher rider outputs. I'm not completely convinced by some other aspects of their methodology, and they were comparing a belt with some narrow bushingless chains, which you wouldn't use on a fixie skidder anyway.
All that said, the numbers are not even big enough to be important in the context of leisure and transport cycling if there are corollary benefits to belt drive which exceed the high costs. For me, the claims of cleanliness and silence don't add up to a worthwhile benefit weighed against the costly components, difficulty of finding replacements in case of wear or breakage and the need to have a custom or customized frame. A waxed Connex 1Z1 is clean and silent when accurately set up with decent quality sprockets, it's cheap, and in a pinch you can usually get an emergency replacement in the sports aisle of your supermarket.
tension requirements to be high enough to damage hub bearings when run fixed. Could be nonsense
Is nonsense. The preload is about 400N, which is also the mean chain tension at 250W/60rpm for a typical fixie skidder gear of 48/19. Peak chain tension would routinely be at least twice that. There is small addition to total pull on the axle as the belt, by design, must never drop to zero tension in the bottom run, but it's trivial. Radial load on the hub bearings from the rider sitting on the bike will be of the order of 500N static, a bit more under acceleration due to load transfer. 6000 bearings, which are about as small as you'll find in rear hubs, will take about 2000N each static and over 4000N dynamic.
TL;DR: Belt drive works, but chain drive works better for less.
gbj_tester
 FTFY