A power meter can provide an extremely accurate measure for gauging the intensity of your workout, but it's also an extremely expensive gadget. Unless you are training hard and seriously for a cycling race, triathlon or other event, this tool is really more of a luxury than a necessary workout aid. Still, it's nice to know how cycling power works.
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Utilize cycling power estimates to gain an idea of how much power in watts you are generating. At 12.5 mph, you'll generate about 75 watts on average. Use these averages: 95 watts for 14 mph, 120 watts for 15.6 mph, 148 watts for 17.2 mph, 180 watts for 18.7 mph, 218 watts for 20.3 mph, 262 watts for 21.9 mph, 311 watts for 23.4 mph and 366 watts for 25 mph.
Approach your calculation a different way if you are trying to determine how many additional watts you need to gain speed increases. If you are expending 75 watts and traveling 20 kmh, or 12.5 mph, you need another 20 watts to raise your speed by 2.5 kmh to 22.5 kmh, or 14 mph. However, this is not a linear equation. Instead, the increases in power you need to increase each 2.5 kmh go like this after the first increase from 20 to 22.5 kmh: 25 watts to get to 25 kmh, 28 watts to get to 27.5 kmh, 32 watts to get to 30 kmh, 38 watts to get to 32.5 kmh, 46 watts to get to 35 kmh, 49 watts to get to 37.5 kmh and 55 watts to get to 40 kmh. The numbers are progressively bigger because the power you need to overcome wind resistance increases with the square of the velocity, according to the Road Bike website. To convert your kilometers per hour to miles per hour, divide the kmh by 1.6.
Disregard both of these formulas if these conditions are not exact: You are riding on a flat road on a day when there's no wind. Avoid using simple power output formulas to plan training sessions. Even in these ideal conditions, other factors will affect your true power output. These include how much your bike weighs, your weight, your positioning on your bike and the air temperature. Other factors that come into play include your wheel diameter, crank length and gear ratio. When you divert from the ideal conditions, the grade of the road you are on and wind have an impact on your power output versus speed.
The best way to gain a precise measurement of your power output is to use a power meter. However, you must be prepared to spend more than $1,000 as of 2011.