Any ways to estimate your own wattage without buying expensive equipment?
Find a steep hill, the longer and steadier the better. Determine grade of said hill using any of a number of online tools. Ride it, record your average speed. Weigh yourself and your bike/kit/bottles, etc.
Go to analyticcycling.com and plug the numbers into “Power, given Speed”. The lower your speed, the less effect your aerodynamics have.