Some thoughts fuelled by own experience:
Most food I once carried by bike was for 9 days. It was only for about 100 km as I had the opportunity to send 3 days worth of food ahead.
This is where almost all of the food was carried:
I also had a small rucksack with me for lightweight bulky stuff. Depending on the days ahead, I would swap the light and bulky stuff (rucksack) with heavy and dense (bike) around in order to being able to carry/push the bike more easily.
Going again to ride roughly the same route. Will be travelling with a bigger tent, bigger stove and my girlfriend so we'll likely be covering less distance. We're going to use our steel HT with Tubus Vega, Drybag ontop and King Many Thing cages, full frame bags, down tube cages, handle bar bags. No rucksacks as the route will be easy to ride and have only "smooth" single tracks, but mostly gravel roads of varying quality.
Power on my first biking trip across Iceland (photos) which had a bunch of km's with no resupply options came solely from a battery pack - 10000 mAh. I could've done with less.
For this next trip we're using our everyday bikes on which we have dyno hubs. I've been using the "Forumslader" for many years now and have ordered a second one, so we are completely independent of power.
For a big trip in November/December I'll be carrying food/fuel for 30+ days, including up to about 14 L of water.
No chance in my point of view to do this without racks. I'm trying to get around a front rack by using 2x triple Mount per fork leg, but not so sure it'll be enough.
The tough part is when you come from long distance riding and visit a remote place - there's no pizza, no bar food, no café at some point(s) of your trip. No petrol station, no Spar, no Morrisons, no Tescos.
This means you can't sit down somewhere and dig in. And then dig in some more. It's very, very different if you can ride for 2 - 3 days, eat a bit too Little perhaps, but then end the 3 day in a village where you can have 2 - 3 meals and 2 deserts and start the next day completely refilled/refuelled or if you just have a pouch filled with 800 kcal of food and bar of chocolate, but not more.
I've done many wilderness trips of 20+ days with no resupply option (well, I could've have had a bush pilot make a Food drop, but think that's ridiculous) hiking, mostly in subarctic climates (just about any you can find in Köppen's classification) where you burn through even more energy and I always get away with a minimum of 650 g of calorie dense food (3500 kcal), despite 10 h hiking/day. Of course I lose some kg of body mass.
Yet if I'm on the bike riding/pushing for 10 h/day, 650 g or roughly 3300 kcal/day is nowhere near enough. That's typically been the case with about 1250 m vert gain in 100 km. 800 g/day is a minimum adding up to 4100 kcal/day.
If the ratio is at 4700 m vert gain in 100 km, loads of pushing/carrying, super steep and tech descents and I go for 10 h, no matter how slow I go - I need at least 1 kg/day.
Another thing to consider with fuelling for bikepacking for 10 days or more, is the frame's rigidity.
Steel and titanium are comfy and all, but generally not ideal for larger loads as they become rather soft. There's a reason why proper touring bike frames are so heavy.
So you need to use stronger tubing (thicker sidewalls/larger diameter), or else the frame will feel like cooked pasta when loaded.
I load my Krampus regularly with up to about 40 kg (car free life...) and it feels seriously soft. Even with 20 kg it feels soft, that's a total system weight of about 117 kg.
On a bikepacking trip on my own that'd mean food for 10 days and 5 L of water. When riding with my girlfriend, food for 10 days and 3 L of water. Upwards from that I don't like the way it rides. I don't like soft (comfy some say…) frames anyway.
That - for example – was the main reason why I sold my Muru ti fatbike frame and had a custom one made. The Muru was already soft with winter luggage and food/fuel for 4 days at probably about 110 – 115 kg total weight. I wanted a bike that feels like the Muru with a total system weight of 110 - 115 kg, yet when loaded with 150 kg. That's 25 % difference. The Muru weighed in at 1700 g. The new frame is a bit longer and by a suggested plus of 100 g (total frame weight of 1800 g) the new frame would have to be 2250 g to roughly give the feel when loaded. Luckily, the frame is just exactly that weight.
Many things I stated are of course subjective but I may give you an idea where my thoughts during trip planning and also in retrospective go to and come from.
Good stuff, I just had read the opening post. Started writing. Ran off. Came back and wrote the rest and realised many had answered before and hey, we all basically go by the same numbers.