I agree with Chaadster, how much you need depends on how hard you are going and what the conditions are like. If you are just noodling around sightseeing in moderate temperature conditions, everybody saying "just water" is probably correct. But if you are riding hard in hot weather, going 2-3 hours on just water will leave you pretty depleted and probably with a massive headache. I prefer to keep my hydration and my nutrition separate so that I can adjust either one according to the conditions and how hard I am going. I like using Camelbak Elixir tablets in one bottle and water in the other. The Elixir tablets are similar to Nuun, but Nuun taste like warmed over cattle spit to me. They have electrolytes, but very little in the way of calories. If I am riding hard for 2-3 hours in warm conditions, I will use a gel or some other 100-150 calorie snack about once every 45-60 minutes to avoid bonking and drink about 1 to 1.5 bottles per hour.