Aerobic power, the most important to an endurance athlete, can only be found accurately from testing. Anaerobic power also called functional threshold power (or your one hour average power) is easier to guesstimate but should also be done through testing. V02 max power is your average wattage output for about 5 minutes of cycling (also given after a test). This is an all out effort and you should have nothing left at the end of this time. These three numbers describe the functional output of your body and where training should be targeted.
Here is an example of a recent test result:
Aerobic power (AP) = 220W Anaerobic Power (AAP) = 270W V02 Max Power (V02) = 325W
Ideally, AP/V02 = 70-75%, AAP/V02 = 80-85%
Athlete: AP/V02 = 68%, AAP/V02 = 83%
After looking at the ratios for this athlete, there needs to be a slight improvement in anaerobic power and more work needs to be done to increase aerobic power. Recommendations would be to focus more on longer easier rides as this will strengthen both these areas. Overall this athlete is getting close to maxing out their current capacity and will have to focus on improving their ceiling.