When talking about human power output, let’s get our terminology straight. If a bicycle rider burns 300 calories in 30 minutes, how much power output in Watts are we talking about?
300 calories = 1255 Joules
30 minutes = 1800 seconds
Power = 1255 J / 1800 s, or 0.7Watt
However, more commonly, you would express this in kilocalories, in which case multiply everything by 1000. The efficiency of the human body is often quoted as 40%, so that’s a power output of 280W, which is not unreasonable according to the chart below: