Don't believe what they all say about "weight".

[GrasB]

I would be skeptical of how well it works! But a few brief tests where I know the true power output suggest that it works better than the power estimator on Strava!

Depends on how close its assumptions are to your actual data.

 

[gam001]

useful calculator here>>>> http://bikecalculator.com/wattsUS.html

at 15.5 stones, level ground
15mph = 110 watts

15.5 stones, 1.5% grade (the same as a well laid garden patio)
15mph = 220 watts
3% = 335 watts
5% = 485 watts

As Rob3rt says, the absolute values are rubbish - e.g. I'd be at around 150 watts on the hoods for 15mph on the flat.
The relationship between the numbers looks sensible though, and illustrates the point well