Earlier this spring I had a training ride with Hans Christian Tungesvik. It was a 4h ride with 15 x 7 min on/3 min easy at 240/260/280 watts, Hans Christian’s watts. On the first 7 min on 240w I pushed 274w to keep up. On the 260w I went 286w and on the 280w I went 305w. On average I needed to push about 25 watts more than HC to hold the same pace.
I am about 2-3 kg heavier than HC but that only accounts for 3-4 watts on a flat surface. We were both riding our time trial bikes and kept as much aero position as possible. What was the reason for the different need in output?
- Hairy legs – I had not shaved my legs. “Internet” claims that cost 10-15 watts, but that’s measured in high speed at 45 or 50 km/h. At the 35-40 km/h that’s closer to 5 – 7,5 watts lost. I could easily put on Bioracer Speed Tubes to fix it, but I didn’t.
Hairy legs do not look very cyclist like and do not perform well aerodynamically either. - Round bottles on the frame – HC had one bottled on the frame while I had one 1 liter and one 0,65 liters bottle on the frame. Aerocoach did some testing and found that a 900ml bottle costed 8,2 watts on the downtube at 40km/h. My two bottles would probably be somewhere around 7,5 – 10 watts depending on speed compared to not bottles and half that compared to HC.
Riding with four bottles seem overkill. However, my rear bottle is a tool bottle and with one 1 litre bottle and two 0,65 litre bottles I can ride hard for 4 hours in without stopping to refuel. 
HC’s race bike was aerodynamically «cleaner» than mine training bike and probably a few watts faster as is. - Light on my helmet – I striped a 100-lumen light on the top of my helmet. Great for being seen when riding in low visibility conditions, but aerodynamically not a good idea. Watt cost? Impossible to say but 5 watts is not unrealistic.
You can see the light on the top of my helmet which does cost watts. Also, my LG road helmet is probably less aero than HC Lazer aero road helmet. Note that in a race I always use a TT-helmet, and so should you. - Wrong tire pressure – I did not pump up my tires before I road and noticed the front tire to be rather soft. When I got home I measure it to be 50 psi(!), which is very low for a 23mm tire. For reference it should be close to 100 at that width. For tire pressure, it is better to be lower than optimal compared to higher, but that would probably cost me 3-4 watts.
- Dirty drive chain – Cost around 2-3 watts.
Counting up everything we are up to 20 – 25 watts.
Honestly, I don’t mind to be the one who has to push most watts in training. I enjoy training hard. For that reason, I am not obsessed about optimizing everything in that setting. For racing, it’s a totally different game. I leave no stone unturned.
What I do see often is triathletes doing these same kinds of aerodynamically mistakes in races. It’s totally unnecessary and will cost your performance to suffer substantially. So invest in knowledge about aerodynamics and clean up your setup and you will definitely go faster. For other advice on how to go faster check out my other blog posts:
Allans triatlonverden 
Riding position probably plays a part too. How aero are you compared to him when pushing those watts? 🙂