Yeah there'll be variance. My undergrad was a While ago but at that time the numbers available were about 25% on average throughout the day, but these numbers are pretty dependent on who was being measured and what their base metabolic rate was.
The thing about the brain, though, is that the brain knows exactly how much energy it needs and that amount of energy is effectively independent of what else the body is doing. The brain has absolute privilege (this is the actual biological term) to take control of the production of energy in the body to ensure that, no matter what, it always gets exactly what it needs to run, and the only time that number changes is when the brain unilaterally decides that it needs to cut its energy consumption, which is quite rare.
Your brain will consume about 25W of power irrespective of what else the body is doing: if your body starts running a marathon, your brain doesn't reduce its energy needs to make more energy available to the body, because your brain is doing the coordinating and it still needs 25W of power. So... that's what it demands. If your body can no longer supply enough energy to support both running that marathon and keeping the brain supplied with 25W of power, the brain will never be the thing that consumes less energy.
This is facilitated by a series of back-channels and alternate control schemes that allows the brain to independently alter things like thyroid hormone production and the operation of the intestines to ensure that high-consumption organs stop consuming energy when the brain wants it more and that your cells start churning out more sugar when the brain needs more of it. On top of that, your brain (and also your eyes, which are an evolutionary extension of the brain) has special glucose receptors that do not respond to insulin at all to ensure that, even when your body isn't producing insulin and thus your other cells aren't consuming blood sugar, your brain can ALWAYS consume as much sugar as it wants. It always gets first dibs on sugar and it always gets priority on oxygen.
Simply put... every animal specialises into some kind of niche. Rodents are small generalists that specialise in being able to eat almost anything and extract as much nutrition as possible from any available source. Bears specialise in being able to avoid predation and chase away scavengers, allowing them first dibs at high-value carrion and vegetation. Cockroaches specialise in being the ultimate detritivore, all flighted birds specialise in being able to fly alongside whatever else they do, etc.
Humans are intelligence specialists. We are, more than any other single organism on the planet, solely devoted to feeding our brain and keeping it as hyper-efficient as possible. Every single part of our body is stripped down as much as possible to make the brain operate better. People talk about us being long-distance runners and like... that's not wrong? But if you look at the diversity of human lifestyles, that's just ONE thing we're pretty good at. None of it matters, though, if humans don't coordinate with each other because realistically one human is not a threat to an antelope. Five humans, each carrying tools that they built to enhance their reach and deadliness, though? Those are a threat to pretty much anything smaller than a crocodile. And the thing that made those tools, that facilitated the coordination? That was the brain!
Humans' biology is entirely fixed to making sure the brain gets everything it needs, whenever it needs it, with zero questions answered. Our intestines are relatively inefficient digesters compared to most similar species because we evolved to strip out redundant features and replace them by cooking food first. Cooking over a fire allows you to stop wasting food-energy on digestion and instead burn flammable materials' stored chemical energy to do the digestion instead. Rather than producing amylases to break down starch molecules... just heat the food using a fire for a long period of time, which breaks down the starch without you having to use your biological energy.
We stopped growing hair and stopped wasting energy on other defensive and thermoregulatory features because we invented clothing. We realised that, rather than growing fur to try to control temperature and resist predator attacks and thorn cuts, we could instead make vegetation and harvested animal hides into something that could cover our bodies, allowing us to select for individuals who didn't waste energy that they could instead outsource. Suddenly, our immune system could stop wasting time fighting parasites because we killed endoparasite eggs by cooking them and avoided ectoparasite bites by wearing clothes, meaning our immune system is much more energy-efficient than it otherwise needs to be.
All of this is to make that 25W number as LARGE as it can be, while the brain strips out functions it doesn't need anymore to make that 25W number as EFFICIENT as it can be.
Don't need complex intestines? Cool, all those intestinal functions are either being outsourced to the gut or being removed entirely. More cranial capacity for the frontal lobe.
Don't need muscles for climbing? Cool, cut that shit, devote all that parietal capacity towards bipedalism instead. Now the hands can take up more sensorineural capacity, allowing the upper limbs to be entirely altered into tool-manipulation limbs instead of worrying about locomotion. Fantastic, more tools for the outsourcing gods.
Cooking food? Using knives? Awesome, stop having claws, they're getting in the way of the fingers' attempts to manipulate fine objects. And while you're at it, you don't need fangs or slicing teeth, two incisors will do and they don't need to be large. Stop growing massive masseters, too, those are a huge waste of time now that we've invented hammers to crack bones and nuts and boiling to make roots soft instead of eating them when they're raw. Awesome, that's a massive amount of protein freed up for the new hyper-efficient spinal cord we've been growing that can outsource some thinking capacity away from the brain.
Every single thing that differentiates humans from near-human primates is because the brain wanted more and so our entire biology became hyperfixated on becoming tool specialists at the expense of literally every other organ system we had.
We're weaker than other hominins (not to be confused with hominids, a smaller subgroup comprising only us and our extinct near-relatives), we're less adept at climbing, we're MUCH less adept at physical unarmed combat, and we're generally far less able to eat raw food or scavenge resources without tools. We're also more susceptible to most foodborne and waterborne illnesses and we're generally just kind of shit at trying to be a chimp.
This is because every single one of those functions has been entirely replaced by TOOLS.
Chimps sometimes, occasionally, use a tool to make a tool.
Humans use tools to make tools that are used to make tools so often that we do not even consider it enough to remember doing it.
We make those 25 watts FUCKING WORK FOR US.
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