fwiw, riding sitting on a spinning bike for 30min i burn about 400 calories at 270w average, which would translate to 800cal/hour. however that's pretty damn intense and i'm not sure if i could sustain this average for an hour, let alone longer.
for long/audax rides i reckon my output average is 150-180w (not tested, but comparing perceived effort), which is 450-500cal/hour. aformentioned 70miles ride is done in 4hr when ridden vigorously (~27kph), so calorie count for me would be ~2000.
what i'm intersted in is if doing different excercises (e.g. running, swimming, rowing etc) at the same heartrate burn the same/similar amount of calories.