The fundamental units of a Watt are m^2∙kg/s^3. That makes this a fine project for an 8051. You'll need to enter your weight (okay, your mass in kg) prior to the ride (and assume it's not going to change), and add the weight (mass) of the bike (assume it's a constant). The only sensor you now need is a speedometer. If you know your velocity you can calculate acceleration. Velocity ∙ Acceleration ∙ Mass = m/s ∙ m/s^2 ∙ kg = Watts.

Good luck,

Joe