Enough glycogen to generate 2000kj of work would be approximately 8000kj (or 2000 kcal) worth of stored energy because you’re operating at 25% efficiency, thus the 1:1 for kcal stored:kj produced. You can’t equate the work you do (in kj) with the energy consumed (or stored) in kj at 1:1.
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So what?
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Sure, and you’re right… it makes the conversation simpler.
But I keep going back to one point: this ALL hinges on how large your muscle glycogen reserves are initially. And that, we have no way to help you with. Is it 1800 KJ? Or 2000? Or 2400? Or 1600 KJ? 
You can gradually get to a number that’s useful to you in practical terms through experimentation. But there’s not much we can do in this conversation. IF your 1800 KJ number is correct, THEN your estimate that at least 45g/hr of carbs should be your minimum fueling level is reasonable assuming you start fueling immediately after the run starts.
I’d still fuel with more than that, but yes it’s reasonable. Again, back to trial and error. Try it and see how you feel after a five-hour marathon with that fueling strategy. Then try it at 75 g/hr and see what the differences are.
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I think I’ve verified that as best I can; when I fall apart in marathons, combined with kj burn estimations from the fellrnr calculator and Garmin records.
Maybe I could do some maths on the lean muscle mass from my scales, but it’s widely reputed these are unreliable and don’t think there’s a way to know liver stores beyond average human numbers.
So on the fuel, 45g of carbs generates 45x16.7 = 751.5kJ or are we arguing that’s 188kJ?
If it’s ~750 I should be fine, if it’s ~190 then I’m still in trouble and I’ll be out of fuel at (est.) 3h20 instead of 2h20.
Sounds about right. Just remember that the percentage of energy you get from carbs rather than fat, increases as your intensity of effort increases, or with mild dehydration, or in several other ways. So you may want to fuel a little more if possible.
I dunno.
45g of carbs generates roughly 180kJ. It’s all been normalized for you, so that’s roughly 180kcals.
And then you realize it’s a range, not exactly 4kJ per gram. And that you aren’t 25% efficient. So you take kJ from power meter and figure out how to adjust intake based on what happens weighing yourself on the scale, day after day, week after week, month after month. And don’t be alarmed if taking in more calories actually results in better performance and weight loss.
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No, 45g of carbs has 751 KJ of energy. Here, we’re ONLY talking about the actual amount of energy in the food.
The real issue is whether that amount of fuel is enough. He estimates his muscular glycogen reserves at 1800 KJ and says he depletes them by 800 KJ/hr while running. He’s asking whether putting in 750 KJ/hr of fuel will be enough.
Those 750 KJ/hr (a.k.a. 180 kcals/hr) reduce his energy deficit and prolong TTE. The question is, is that enough? I’m not sure it is, but it might be IF his numbers are right.
As noted several times, body efficiency varies within a range. And carb/fat utilization varies with intensity, hydration, and other factors. And honestly, I think doing all of this in joules is complicating things unnecessarily and introducing doubts like the question you’re asking here. But I understand that you want to work through the exercise, so have at it. 
Not sure I can add anything of further value to this conversation. Hope somebody has more expertise on the matter and can help us all get this clear.
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Omg guys! 
Please for the love of god just put some snacks in your bag and go for a run. When you feel hungry you can eat them.
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Say that to me when I’m crawling the second half of a marathon and see what response you get…!
If you think I can just pop out for a 3h20 run to see if a handful of jelly babies is good you’ve got another thing coming 
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Last Saturday I did an almost marathon length training run. I knew they’d be food at the halfway point, my local parkrun, so I took no supplies, not even water. (Was reading Born to Run at the time so full of the romance of the Tarahumara).
At this stop I had two cups of tea, a glass of milk, two chunks of cake and two flapjacks. Then I ran home again.
It went remarkably well. Was zooming along towards the end.
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I drink every Tuesday with a bunch of sub 3 marathoners, and other club runners, great guys - my given nick name is ‘Slim’…
I’m not taking their nutrition advice though, for example; Sunday, London marathon, 2h42, recovery ten pints a short walk from the finish line, Tuesday 1hr at <4:30/km, followed by recovery jugs of ale…
I train about twice as much (swim bike run) as they do, but they’ve been runners all their lives. Maybe I should quit solid food, about the only thing I haven’t tried in the last ten years. Pale ales and gels, what could go wrong. 
Lots of calories in a pint 
(I rarely drink btw)
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In unrelated news a friend mine is in Medoc (France) at the moment. I don’t think he’s got my marathon references yet…
I did a half last year inspired by the Medoc. It was scorching hot and we were pouring water over ourselves as well as drinking wine at each of the 6 aid stations. It was brilliant. Could’nt feel any discomfort by the end, although was an hour slower than normal 
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My Lord, well done sir!
I’ve been agitating for the club to instate the end of season beer mile but for a drinking club with a running problem they are sadly not biting.
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Your rough estimates align with the usual 45-60g of carbs per hour. I’d suggest pushing carb consumption during the marathon a little higher, closer to 60g/hour and see how that goes. If your metabolic fitness is lower, you might find even more (70+) is better.
So easy to get lost doing quick math using kJ and kcal - I’m not the only one making mistakes on the quick & dirty calc. You’ve got a rough estimate now, ready for refinement by testing it out on the road
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So on the fuel, 45g of carbs generates 45x16.7 = 751.5kJ or are we arguing that’s 188kJ?
I’m saying 45g of carbs generates 750kJs, and that allows you to do 190kJs of work. (They body is roughly 25% efficient)
I believe others are talking about the ratio you burn carbs and fat, which is relevant, but you are not replacing all energy burned if you’re only eating 45g of carbs. 45g/CHO may be enough to get you through a 3hr race without bonking, because you burn fat as well, I just wanted to clarify what I’m saying.
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