A professional cyclist can steadilly output 400 to 450 Watts of power.
A fit cyclist can steadilly output 200 to 250 Watts of power.
A casual cyclist can steadilly output 100 to 150 Watts of power.
Q: How to determine the amount of power you can steadilly output?
A:In my case, I went to a gym that has an indoor exercise bicycle with a display of speed, power, distance, etc... I set the display to indicate the power output and started gradually increasing load to the bicycle until I reached my comfort limit. At that point I was able to steadilly output around 150 W. By steadilly, I mean for at least 1/2 hour. The more you cycle, the fitter you'll get and that limit will increase up to a certain point. You will most likely do better with a real bicycle as the riding position, saddle and handlebars will give you a better fit.
A more accurate approach would be to try the hardest steep hill that we can comfortably climb (we need to know the slope of the hill that can be obtained in websites with a map that also show the elevation, like www.mapmyride.com ) and record the speed (the climb should be done at a constant speed).
The following article on the Copenhagen Cycle Chic blog has some good cycling tips, unfortunatelly the point 5 (gears) can be misleading since it tries to apply flat city arguments to hilly cities. And even though I agree that the bicycle's weight is not signifficant up to a certain point, it is not totally irrelevant either. The whole weight (bike+cargo+cyclist) will be something to consider when chosing the gearing range of a bicycle, except when that bicycle is to be used only in flat areas.
To overcome some steep hills when this type of infrastructure is not present, one can either walk the bike up the hill or have a lowest gear that is low enough to make it within the rider's available power. It's not enough to say that "if 3 speeds don't make it get 5 speeds". You could even have 18 speeds and still not be able to climb that hill. In this case what matters is how low is your lowest speed and how much weight will you have to carry uphill.
This weight will be the total weight of the set bicycle+rider+cargo, so, lets see what happens if we have a 70 Kg person riding a 13 Kg bicycle carrying 7 Kg of cargo (total weight 90 Kg) and we add 5 Kgs to the bicycle's weight.
These 5 Kilos can mean 1 set of fenders, chainguard, mudflaps, rack, basket, saddlebag and set of tools which would turn an inadequate commuting bicycle into something much more practical.
a difference of 2 Watts which is not even noticeable by the rider. Also, the power needed to carry the load is small to begin with because it's an almost flat surface.
As you can see on the graph above, even if the total weight raises to 160 Kg the power needed to carry it up the 1% slope will be under 50 Watts. In these conditions anybody can ride even a fully loaded cargo bike.
the 12 Watts difference could barelly be noticed by the rider, and would it be worth saving weight on a set of accessories that make all the difference when using the bicycle for transportation purposes?
I still don't think so. But what we see is that the steeper the hill, the bigger the difference even a small 5 Kg load will make, because now, the necessary amount of power to carry the load up the hill is aproaching the power we can steadilly output.
1-Lowering the gear ratio: replacing the rear sprocket by a bigger one or the chainring by a smaller one.
by using this sheet, you will see that reducing the cadence will translate in a decrease in the necessary amount of power to go up the slope. This happens because you're lowering your speed and Power = Force x Speed. However, the force you will have to apply to the pedals (Avge. crank force) will be the same and if this force goes beyond your comfort level, reducing the cadence will make it even more painfull on your knees. In my case, I feel comfortable pushing the pedals with a force up to 20% of my weight because I never get up from the saddle to push the pedals.
If you usually stand to push the pedals, then maybe you can use your whole weight for a while. If it's a long climb you will need to do something to the gear ratio.
The following graph shows that for an 80 Kg set at a cadence of 60 rpm with 39 gear inches most people would be able to climb hills up to a 6% slope (under 150 Watts of power required).