Using an online speed-to-watts calculator there is a big differeence between riding 15 mph into a 15 mph head wind vs. riding 30 mph into a 0 mph headwind.

Assuming the rider is 150 lbs, the bike weighs 22 lbs, clincher tires, the same crank length, and the rider is in the drops. A rider hammering for 10 minutes @ 30 mph will average 499 watts. The same rider traveling on the same setup riding 15 mph into a 15 mph head wind for 10 minutes will average 319 watts.

It was a free online calculator so take it for what it's worth.