3000 kcal is not double but what your average, active male will consume. I know because that's what I consume. Do you mean from physical moving of objects alone? I don't have the time to check your other numbers, but I wonder if they are correct. Additional fuel is trivial btw: cellulose and so on. Have fun.

How much super do your superhumans get to be? If they are only lifting semis, we can work out something biologically. If they are lifting ocean liners, we need to look for a different energy source.

@Alexander For now, I'm trying to just get them to lift semis. I'll edit the question to reflect that.

Two prolems with your question. First, why would they lift a semi 50 times a day? Weight training? Second, most of that ~3000 calories consumed by the typical male does not go towards lifting stuff. 2000-2600 go to basal metabolism: healthyeating.sfgate.com/… It's not unusual for a fit & athletic person to consume much more than this. So the extra ~3000 calories does not seem to be a big deal.

I am not sure of the policy here of stealing ideas from existing fiction, so I haven't posted this as an answer: "Solar energy from the yellow sun is the source of energy for Kryptonian powers. " source

As a reference for @Renan's comment. Michael Phelps would eat as much as 12,000 kcal a day. So it's entirely reasonable for your superhuman to eat like a hippo.

If you are interested in a science based answer you might like to read about Yoda's power output (20kW) what-if.xkcd.com/3 and an amusing comic that suggests Superman's best use to humanity is turning a crank to produce free, clean energy smbc-comics.com/index.php?db=comics&id=2305

Half a cup of butter is a bit over 800 Kcal, is there an issue with butter wielding superheroes? In fact, heroes that eat oils and other high-fat content foods would be more like mountain climbers.

Eating like an athlete is not eating a normal amount of food, actually. Most athlete spend their day eating when not training, meaning they do not really have much time for anything else. So it would not be considered eating normally to eat as much as an athlete. Or even more.

@fredsbend: But I have never seen any study showing that the brains of intelligent people consume more calories than those of unintelligent people. Intuitively, I would think it'd be the other way around: being stupid is hard work. (Compare computer CPUs: each new generation uses less energy per operation - see "FLOPS per watt".)

@James First, computers are not like brains, despite the constant repetition of this. Efficiency of an organ doesn't always correlate to total required load in biology. Some systems are stupidly inefficient because evolution is messy and has no aim other than proliferation. The difference in brain energy consumption between intelligent and simple is probably negligible, but the act of thinking and learning intentionally surely uses more, just as lifting the semi does.

I'd posit that the intelligent depend on their brains more. A super-intelligence has a more powerful and active brain, just like the human brain is more powerful and active than a monkey's. Ergo, the super intelligent would require more energy for their brains to operate.

@AzirisMorora Eating does not take that much time. You can eat specific foods that are higher in kcal value than others without having to spend twice as long or three times as long eating. Yes, athletes do eat a lot of food, but they still do it in about the same amount of time as avg folks eating food (e.g. an hour for each meal, maybe snacking throughout the day as they do something else).

@fredsbend: I don't agree, and I doubt there's any actual evidence showing that intelligent thought consumes more energy than the unintelligent variety. My own observation suggests that it might actually be less, as thinking, like many physical skills, seems to get easier with practice.

Because you'd get shitty scenes like this.

Work it into the story like this: shadowunit.org/readingorder.html

Larry Niven's Known Space universe includes a race called the Pak, and in their third stage of life they achieve superhuman strength via the "swelling" of their joints, which (is as oft-repeated) increases the moment arm of said joints. I always appreciated the plausibility of that mechanism and the principled effort to conceive it.

Also consider how the semi is going to be lifted... There is not a hand-sized area on a semi that would be able to hold it's entire weight. A big problem I've seen with "super human lifting" is that the object being lifted would simply buckle under its own weight or be severely deformed by the small pressure points.

@james You are right. Brain energy consumption is largely constant despite use. Further, evidence points that the mentally disabled and savants do not use more or less energy in their brains than average persons. Perhaps, and this blows my mind, intelligence is not at all correlated to energy consumption. However, since I mentioned evolution's examples of poor efficiency, perhaps our brains are stupendously inefficient and could be better.

@fredsbend: Perhaps I'm prejudiced, but my guess is that more intelligent people don't actually do more thinking, they just do better thinking. To go back to the computer analogy again (however imperfect, it's the best I have), they just seem to use more efficient algorithms to get to a result.

@JPhi1618: Actually, there are roughly hand-sized areas on a semi trailer that could support its entire weight. Anywhere on the frame, for instance. It's not uncommon to lift a trailer with a large forklift or a jack, and the contact area is not that much larger. Of course if your superhuman is trying to lift tractor and trailer, it will flop around at the hitch. And doubles are going to be even worse :-)