Training & Nutrition - Calories per hour on century+ ride?

I am wondering if anyone has any references on how many calories per hour I should be consuming while doing long distance riding? I'm a 210 lb male. My typical speed for such a ride would be in the 15-17 mph range. On each of my century rides, thus far, I get to about the 60-80 mile mark and feel lethargic/dead legged. I usually take break, eat some more and recover reasonably well. I would like to try to avoid this in the first place. I understand I will still have to experiment some, but I am looking for a starting point. I feel like I have been guessing so far and not guessing particularly well. I would appreciate any thoughts.

It's somewhat individual. I weigh 200#, and I target 150+ calories/hour on hard rides: faster pace than you plan, but usually shorter. I'm doing a century tomorrow, and will use the same target. It depends in part on what you can stomach, which in turn depends on the type of calories. Some calories are more easily digestable. I've settled into using Sustained Energy, with a scoop for every hour I plan to be out there (~115c per scoop? something like that), supplemented by a shot of Hammer Gel (from a flask; ~100c per hit) just before any harder section, or if I think I haven't been drinking enough SE. I usually end up not getting as many calories as I had targeted from the SE, especially on longer rides, which is why I supplement with the Gel. Bars are okay, but they are so dry it's hard for me to choke 'em down while riding. I don't want to stop if I have an elapsed time goal, which makes it hard to have enough calories in bottles and also cover hydration - I need a backpack to do it. That slaos eliminates the 'real food' they may have at the SAG stops. No rush tomorrow, so I'll stop for refills and probably have a banana or something like that, just 'cause it's nice to have something real in my stomach.

Your feelings at the 60-80 marke probably have as much to do with muscular endurance as with nutrition, but you certainly want to eliminate nutrition as a limiting factor. IMO, ~150 calories per hour should do that.

It's a personal decision. Some people can process up to 400 Cals/hr. Studies have found athletes have better performance over a 100km time trial by consuming a drink with both glucose and fructose than simply glucose. Something about additional pathways for energy delivery. An isocaloric glucose-fructose beverage's effect on simulated 100-km cycling performance compared with a glucose-only beverage. (http://www.ncbi.nlm.nih.gov/pubmed/20479485)

The only way to tell is to experiment on some longer rides and see what works for you.

Aim for 200-300 calories per hour consumed in liquid form (sports drinks, coke, orange juice, etc.), gels, or solid food.

What you consume is up to you because people like different things. I like things like chocolate chip and raisin cookies, salted almonds, granola bars, etc. but those might not sit well with you, and you've got to experiment to find out what works.

Here's a resource you might want to browse through: http://www.ultracycling.com/nutrition/nutrition.html