Officially, 6 volts AC, 3 watts.
Unofficially, some folks claim that with home-brew electronics, they can get 10 watts out of them. Still 6 VAC.
Not knowing much about that particular set up, but I'd think if you could get 3X the power out of it, you'd be running the components in the hub in a manner that would contribute to their early failure.
J.