Postulate

My world has functioning magic which is capable of exerting kinetic effect. (That is, move objects, or generate heat, since "heat" after all is just motion at the atomic level.) It can't directly affect electromagnetism (i.e. light, electricity), but magic can be used to make a generator spin. (Invisibility is right out. Likewise any sort of "psyonic" powers... no mind reading!) Hopefully I don't need to go into too many details; suffice to say, there are useful applications of this magic.

Critically, however, magic does not create energy from nowhere. I'm following the Eragon model; the energy to do Work (capitalization denotes use of the physics term) using magic has to come from a living creature. (I'm not sure if I want to allow plants, partly as I'm not sure it's even useful; see How much energy do plants produce?. For now, let's focus only on animals. Note also How much can a magician lift if constrained by her own body's energy?, Magic and physics with human power output and How can wizards do such powerful things running on pure human metabolism?.)

One important caveat is that there is an "efficiency loss" that is relative to the distance between the "energy source" and wherever the Work is actually happening. For practical purposes, just say that you don't want these to be more than a few meters apart, if that. Another is that there is no known way to "store" magic (at least not for long periods, "long" being "minutes to hours or longer"); whatever power is used by magic has to come immediately from a living thing and at least partly requires metabolic processes (i.e. there is a limit to how quickly energy can be used).

Question

A human can provide maybe 75W continuous (indefinitely), 250W for maybe an hour or so, or 1000W for about a minute. This, however, requires that the person sits next to whatever magic device they want powered. This makes something like a magic-powered washing machine plausible, but inconvenient. (Note: yes, I'm aware trained athletes can produce more, but using magic tires the user just like exercise. I think these numbers are plausible for what an average person would be comfortable supplying.)

How plausible is it that this society would (or would not) develop and use some sort of living batteries? I'm thinking about some creature that they would keep around whose sole purpose is to sit next to magic-powered devices in order to supply them powered. If a magical device does a better job than an electrically-powered version, can the hassles of a living creature (which has to be fed and produces waste) be sufficiently minimized that these would be desirable? What would such a creature be like?

Let's assume that this society has similar views on animal rights as our own, i.e. they're sort of on the fence as far as modern "intensive farming" practices, and most individuals don't want to engage in anything they see as "abusing" an animal. I'm also specifically interested in these "living batteries" a) for personal use, not e.g. what a big factory might do, b) in "first world" countries, i.e. that have developed infrastructure, wealth, a reliably electrical grid, etc.. In other words, would they be relevant to the average suburban American-analog in their day to day life? (If they would surely exist, but would never be seen by an average person, that's not relevant for my purposes.)

More Notes

• Non-sophonts (feel free to read that "non-humans") can't "do" magic on their own. They can only be tapped, intentionally (not by accident), to power a spell set up by a sophont.
• If there is such a thing as "magic potential", it is effectively athleticism. Some creatures may be better suited to producing metabolic energy, but there is nothing inherently "magic" about this.
• "Magic glands" are not a thing. "Giving off" or "emitting" magic is not a thing. Overly energetic animals exist, just as in the real world, but nothing "forces" that energy to be burned as magic rather than, say, running around. (OTOH, parents are totally going to use their kids as power sources ðŸ˜‰.)
• "Magic" is best thought of not as a force in and of itself, but rather a process by which metabolic energy can be directed to do Work. A spell is a means of triggering this Work to happen, which then triggers a designated "supplier" to metabolically produce energy (much as if the supplier was exercising). The "designation" can be e.g. a specific person (a spell that only works for that person), "any living things within range", "whoever is touching the spelled object", etc.
• As a corollary to the previous point, "accidental activation" is not a thing.
• Powering spells is not naturally pleasant. If the power draw is enough for the source to notice, at best they'll feel tired. Basically, it feels like doing work. A natural animal may or may not appreciate this; it's plausible that a purpose-bred animal will.
• Because it relies on metabolic energy, magic isn't much better, from a raw power standpoint, than muscle power. Magic is mostly good at precision. Assembling a mechanical watch? Magic! Hauling freight? ICE or DET! To wit, assume that the industrial revolution still happened, and the world has technology similar to our own, in addition to magic. (With some exceptions; for example, push-behind mowers don't exist, and I suspect household dishwashers don't either.)
• When considering if something like a pig would make a good "battery", keep in mind that a) electricity and natural gas exist (see previous point), and b) you have to feed and clean up after the pig. (This makes the question of locomotion of a living battery a balancing act. On the one hand, you don't want it to move while it's supplying power. On the other, it sure would be nice if it's house-trained.)
• Using magic to generate electricity generally makes sense only when you don't have another source of power... for much the same reason we don't generate electricity using generators turned by animal power. For municipal generation, the same sources we use in our world are simply more productive. For someone "off-grid", solar panels and wind turbines are less hassle... at least if you can afford them. Magic-electric generation is useful for small devices where mains power or batteries are inconvenient. For example, your phone/tablet might be magic powered. (At least you will have external chargers that are magic powered. I'm not sure if they can weigh little enough that device manufactures would want to build them in. Possibly some devices â€” especially wearable stuff â€” will, and some won't.)

To an extent, what I'm trying to figure out is if there are tasks that can be done so much better by magic than by technology, but can't be powered by the person doing the task at that moment, that first-world people would put up with the hassles of a living battery. Laundry is a good example. Modern washing machines (and for my purposes, I'm assuming these at least have the potential of being invented) are convenient, inexpensive to operate, can wash a bunch of laundry at once, and do a pretty good job of cleaning. Magic, OTOH, can do a near perfect job (imagine being able to pull the dirt and whatnot directly out of the fabric; it takes a lot of discrimination, but if you have that, it takes very little force), but someone or something has to power that process. Maybe the machine is used for day-to-day use, and magic is used on really nasty stains? (Or in regions with little fresh water...)

This is also why I believe residential dishwashers won't exist. Instead, your sink is spelled so that whenever you run water over your dishes, all the dirt (and pathogens!) just runs off with the water. Since this needs very little force, it just draws power from whoever is rinsing the dishes. (You're almost certainly spending more energy holding the dish than the magic that's cleaning it.) The same spell could make all the water fall off when you're done, leaving the dishes clean, sanitary, and even dry.

• A funny thought about this: Probably in your universe, on the magical equivalent of StackExchange, they are posting "imagine our animals didn't run our appliances. Would we build a giant grid series of waterwheels, or maybe... I dunno, try to get electricity to run our appliances" "no, that would be impractical, what are you going to do, run wires all over the place, don't be ridiculous." Nov 8, 2019 at 23:12
• I'm assuming that the industrial revolution still happens. There is an obvious reason why it should; we now take for granted motors that produce well in excess of 1 â€” even 100 â€” horsepower. Technology is able to produce far more raw power than my magic can, and that was by design. (In fact, for this reason, factories will probably never use living batteries even if individuals do; technologically-derived power is simply far more efficient at large scale.) Nov 9, 2019 at 1:03
• Your wattage figures are too low. Professional athletes have much higher numbers and a trained mage should be considered a trained athlete in this respect. E.,g for cyclists a 300W average over a 4-5 hour run is considered good (not exceptional) for a trained amateur. Professionals do 500-600 on average. Peak power for some professionals (like climbers and sprinters) can go up to 1400-1600 Watt for 5-10 minutes at a time and around 1000 Watt for longer periods (20-30 minutes). Nov 9, 2019 at 11:14
• @Tonny, citation needed? Wikipedia gives a lower number of 900W for "briefly". Also, I'm mainly concerned with normal people, not ones that are (the equivalent of) trained athletes. That being the case, and also considering efficiency losses, I think my numbers are reasonable. (Also, keep in mind that someone pumping 300W of magic is going to know they're doing so; it's going to make them tired just as if they were exercising hard. IOW, over 250W, people would prefer some other energy source.) Nov 10, 2019 at 15:43
• Hmm, after reading later updates to your question, you force me to consider much more interesting avenues... To make a distinction between your description of magic and how I view it in general, I refer to magic simply as that which is beyond our understanding. To that end all things tiny, distant and conjectural qualify. Why not use magic, atom-by-atom to create fusion? As I've heard energy conversion described in many lectures, a ball sits atop a hill, but in a small valley atop the hill. With a proper nudge it rolls far down, giving up much more energy than it initially received.
– Nolo
Nov 13, 2019 at 13:28

You might think in terms of a bio-reactor for practical purposes. Could be anything along the lines of a compost container. Can be fed any kind of biological material or waste. Can be constructed such that it could be worn. A smart design would be shaped like armor but hollowed and reinforced in separate pieces, each similar in construction to the fuel tank inside the wings of an air plane. More or less complex depending on the mobility requirements of the wearer. A mage wouldn't need to move much but would rather be focused on casting ( and would need the extra reactor volume ), but a wizard warrior would want to be lighter and faster, fighting in conjunction with minor incantations.

And finally, bacteria, the primary "organism" of the reactor is much more efficient than more complex animals at metabolizing organic waste. A simple source of fuel would be the blood of your enemies, easy to manage, can be funneled into a small opening, can be stored, but is impractical for volume to weight ratio. Some desiccated bio matter would be more practical for transporting fuel, this is assuming that water is available in the environment for making a solution of the dry powdered form of fuel ( any dried, ground bio-matter ).

• "Can be constructed such that it could be worn"... I doubt it produces enough energy to be worthwhile, vs. just using the wearer's metabolism. (Er... you mentioned armor. Okay, maybe there are military applications, but for my purposes, I don't care). That said, for stationary applications, I'd started thinking along these lines independently. Accordingly, you get a cookie (and a green check mark.) Nov 10, 2019 at 17:59

Yes.

We already have living appliances, if you want to be really flexible about your definitions. A cat, for example, is a living rodent destruction appliance. A dog is a living alarm system and hunting tool. And, due to the empathetic nature of humans we still keep these appliances around, despite the fact that they are obsolete in many cases.

Dealing with their flaky nature: Many tasks aren't actually latency-sensitive. One idea from "smart grid" tech is that you don't really care when your load of laundry gets done, as long as it gets done before you get home. So, if the grid was able to signal your washer and say "we have excess green energy currently, run your load now at a discount," we'd probably use that. Your magic cat could have a similar setup.

Make sure the most comfortable cat bed you have is on top of the dryer, and it will lay there. In fact, due to selective breeding, the cat bed on top of the dryer is actively the most comfortable spot for your magic cat to sleep, because the magic glands on these things produce more magical field than they can comfortably expend otherwise. So they get grumpy if they don't have an appliance to fuel.

How did this come about? We already had cats for the reasons we have them in real life. We already had manually powered magic appliances, from the beginning of civilization. We noticed that cats were accidentally triggering them. People get pretty creative when laziness is on the line, so appliances were dumbed down to the point where these accidental triggers were enough to run them. The fact that cats now actively seek out the appliances is the natural result of breeding for more and more power-producing cats. Eventually this overcame the normal magic dissipation rates.

Other notes:

• Cats are actually more efficient at producing power than humans on a pound per pound basis, because we expend a whole lot more energy "thinking" and "walking around."

• Stray cats are somewhat dangerous, or at least mischievous -- they are constantly producing excess magic, so they burn it off by performing little spells (small teleportations, pushing stuff off tables from a ditstance, etc).

• You might prefer to use a dog to power your water heater, because they are more easily trained and, being larger, produce more power. In fact, in a rural area you likely have a couple working dogs of different breeds. Guard dogs have had the most magic bred into them -- they tend to stick around the house, so they can power more appliances than herding or hunting dogs. Of course, all dogs have been bred to produce a bit more magic than normal animals, which they use to perform minor mind-reading spells on humans (the human willpower can naturally defend against this, it is an intentional communication channel). Another reason to breed more magic into guard dogs is that, while most dog magic is complementary to humans, guard dogs have an adversarial relationship with thieves, so they need to detect invisibility spells, etc.

• I'm not sure what breeding will do; that's part of the question. For something the size of a cat, however (say, 5-8kg), 20W seems like a stretch. That's nowhere near enough for something like a house water heater. I'm not sure it's enough for a tea kettle... (Also, I think you should re-read my description of magic in my settings... for one, invisibility is certainly not one of the possible effects.) Nov 9, 2019 at 1:14
• @Matthew And that's why Witches are known to have many cats at home. Nov 9, 2019 at 10:27
• @Matthew I don't think we can tell you what breeding would do. I would assume that an animal can put some % of it's metabolism into magic generation, and can also store some quantity of magic. A also like the idea that excess magic must be off-gassed, because it leads to hijinks. It seems pretty reasonable that breeding would be able to tinker with any of those numbers. And that a domesticated animal would have the extra calories (compared to a wild one) to support some tinkering there. Nov 10, 2019 at 3:37
• As to the exact power constraints, clearly these exist but I don't think they are fundamental problems. Water heater might have been a bad example, but if nothing else a dog could supply power to pump water from a well to a small water tower on the roof, to supply indoor pluming (paint the tower black to warm it -- another import from green tech, off-grid green tech seems like a good source for you to mine). Nov 10, 2019 at 3:42
• Off-grid "power" may be the most feasible use. I suppose you could power something with the output of an old hand pump, but no more... Nov 10, 2019 at 15:53

Yes, they would. Doubly so if natural magic batteries are the most efficient form of power that is able to be harnessed.

It will depend on how well this magic is known about and integrated into society as a whole as well as the world's resources

Magic Notes

Since this magic is kinetic based, there won't be invisibility spells to see through on a conventional thought level, but I can picture spells to apply the correct kinetic force to stop other things from moving under natural forces and spells to gently push out and find things that way. Sound is just vibrating air, so that is technically kinetic if I understand right.

As an extra note: If you can use magic to spin a generator, then in theory, you can create a rechargeable battery system powered by magic turning the generator and plug things into that battery as opposed to direct magic powering.

I don't see anything preventing that as per the question, but that would incur a second batch of energy loss so probably not worth it? Technically it's not storing magic but the result of the magic.

Anatomy

Animals get their dietary needs as food. As magic eats into this energy store, the more effort they put into living the less that energy is available for magic.

So for a pure battery, the ideal would be animals that eat a lot but don't do much. They will not be as fat as some would think since while they might not be exercising much, all the energy what would go into getting fat is instead burnt off into magic. In addition, they'll have either more efficient metabolisms and/or just eat more than their mundane equivalents.

Unless an animal's society is already based around a generator-type animal, then pure battery animals will almost always be domesticated animals.

Travelling the Animal Kingdom

Oh the Humanity

There is already an animal that can have the job of running appliances and doing menial tasks that some humans do not want to do -- Humans. Maids, butlers, housekeepers, and other similar professions run washing machines, vacuum cleaners, stoves, and other appliances in order to keep their employers happy.

Depending on the magical output needed, I would foresee workspaces being rearranged so that a magical charge can be applied while a low-impact mundane task is done -- charge the stove while cleaning the kitchen as an initial thought.

While definitely a less sunny tone to the world, there is nothing preventing people with higher magic potential but lower wealth to be servants of the wealthy. If you want an even darker tone to it, nothing says that people can't be bred for a magical specialization. Our laws can be surprisingly cruel to us as compared to others in the Kingdom Animalia.

Travelling down this rabbit hole can make a story go dark fast. Travel at your own risk.

Green Domestication

Just as we domesticate animals for a purpose, magical output and/or efficiency will be something bred for as well. Perhaps it's a primary thing and maybe it's secondary, but it is a trait.

Livestock could be bred to constantly emit small, but useable quantities that could say turn a generator or directly power lights, a heater, etc.. A pig barn that could largely power itself at the cost of some extra food seems like a decent start. As the question was on a personal scale, I'm sticking with a small-scale farmer that might have enough to say power their barn without needing external power from the government.

Zwuwdz brought up domesticating cats and dogs not only for their primary purpose, but for powering things while they sleep or keep watch so I'll skip that one.

Even prey animals such a rabbits could have a purpose. With predators evolving to try to find prey through guile and stealthy magics, prey has evolved to try to find predators so thy can avoid them. Rabbits on edge use their innate magic and keen hearing to detect the smallest changes in the air. These adorable fluffballs become thief detectors and they will work for carrots. You might need a more vocal animal though.

Wild Things

I would also expect wild/feral animals to be carefully monitored around sensitive magical things lest they set off the gadgetry. Some wild animals will be studied for their interesting uses of this kinetic magic, and how we humans can apply it either through attempted domestication or replicating the magic.

And Magic Honey Badger still won't give a damn.

Daily Life

If well-integrated, I would imagine that depending on the level of magic-powered things in the household, pets would be more popular. Money that might get sunk into paying for power or fuel will instead go towards extra food. Pets would also give us humans more things to cuddle and love possibly leading to a minor increase in mental health.

There might also be a small subset of people that sell their time sitting down near some magic gadget to charge it for somebody else while doing a thing -- knitting, gaming, conversing. A gig economy where people can earn some extra cash powering things to keep their surplus energy down (and stay trim).

I would expect to see some jobs that use low-power tools and limited activity to be purely person powered. A tailor running their own sewing machine, a handyman powering their own small drill, etc.

Critical systems will either be powered conventionally or through magic turning a generator as opposed to directly magic powered. This prevents a stray cat or feral coyote from wrecking the highly sensitive thing by existing.

• Pigs, +1. The Matrix would run on pigs; they're about the largest animal you can have with a really high metabolism that can be kept confined and overcrowded into about two square feet per, and they eat garbage. As a bonus, they don't stop work and start asking existential questions. Nov 9, 2019 at 19:25
• +1 for the "gig economy" bit. I'm not sure if I'll use it (read: if it will matter to the story), but I think it makes sense as background. About sound; yes and no. Some forms of precision are really hard with magic, plus, you have to understand what you're doing. A novice could probably produce a simple sine wave, but something like recognizable speech, if it's possible at all, would require years of practice. Again, probably won't come up in my story, but magic ventriloquism might be a thing. (Magic noise cancellation definitely is, and I believe will appear...) Nov 10, 2019 at 17:41

So the thing is, our "modern" technology is mostly about cheap energy.

If you had to feed the animals to get the calories to produce the effect, most of our machines are ridiculously inefficient at what they do.

That 75W that a person produces? That comes from their food. If you redirect it into magic, they need to eat more food.

The energy coming from physically lifting clothing and washing it, or powering a magical washing machine? One would assume doing it physically would be about as easy, as is true in our world. But what we have is a whole pile of energy that doesn't come from farming/sun/etc that we exploit, shove through the grid, and convert into useful work.

On the other hand, energy is rarely a limit on what we want to do. We exist in an information revolution; and that is less about more energy than it is using energy in more efficient ways.

It actually takes almost zero energy to move dirt off clothing; the energy of the dirt on the clothing, or next to it, is nearly the same. The work we do when cleaning clothing comes from converting low-entropy energy (electricity, ATP, etc) into high-entropy energy (heat) while we attempt to rearrange the matter (dirt) so it isn't where we don't want it to be.

The act of carefully selecting every bit of dirt and moving it somewhere else is an act that fundamentally costs entropy to do. Telling something dumb to do it would require insane amounts of "work" (this is, in essence, what computer programmers do); so what we do is do it statistically using crude methods.

One important caveat is that there is an "efficiency loss" that is relative to the distance between the "energy source" and wherever the magic "work" is actually happening.

Note that this is also true of most technologies we use. Electricity in a wire is what we do in order to guide the EM forces down the path we want them to.

Have you considered ley lines? Life would emit your magical field, which would in turn be "captured" by ley lines and transmitted over long distances. They would be much like a power line.

This would be a completely natural phenomena in this world; so you'd get "ley line" building for artificial reasons. It may be impractical for this to occur over long distances, so mayhap the house would have a "ley line ring"; a "magic circle" as it where, permitting magical power to be redirected by modifying glyphs.

I'm also specifically interested in these "living batteries" for personal use, not e.g. what a big factory might do. In other words, would they be relevant to the average person in their day to day life?

Specially bred domestic animals would be probable. Today, even with knowledge of animal welfare, we have people eating hamburgers all the time; in the 18th century people who knew it was wrong kept slaves. "normal" makes something horrible pedestrian.

Your civilization would breed animals who suck magic out of ley lines and emit it locally. They would originally be "magical" animals (unicorns, pixies, dire wolves, etc); the process of domestication would remove their magic-powered defence/offence abilities and get them to be emitted.

We might even take the cow route; maybe they naturally emit excess magic when they have young, and the young in turn 'feed' off that. So we'd induce a constant state of post-birth in these animals to get them in their "emit magic" state.

Now, ley lines' mana is not free. So you'd "enhance" them by rituals and animals and magic circles to pour mana into them.

So in this model, we have

1. Transportation links and settlements along ley lines, so they can access that background magic.

2. Domestic "animals" that are based off mythical creatures -- dragons, unicorns, pixies, wisps, even goblins. They would be bred and biologically manipulated to be in a "emit excess mana on demand".

3. Houses and settlements would be surrounded by magic circles to contain and route magic around from place to place. You can use mumbo-jumbo why the circle is the only "artificial ley line" we can make, and longer ones are exponentially harder to build.

4. Magical experiments involving huge magic circles that feed magic into/out of ley lines would be the state of the art.

• "The act [...] fundamentally costs entropy to do."... and this is where my "magic" is magic ðŸ˜ƒ. The first half of your answer doesn't really tell me anything I didn't know, but you gave a great explanation of what I merely insinuated. For that, you deserve more upvotes than I can give you. As for the rest, basically, there are a few things I want to do in my story that can't be done with science, but otherwise I am trying to minimize the effect of magic, because I want my world to be as close to the real world a possible. (Also, note edits I've made to the question.) Nov 10, 2019 at 17:59
• @matt entropy is important here. Energy is actually free; there is enough energy in a 1 kg rock to throw everything on the island of manhatten 100 km into the sky. It is ordered energy that is of use. Humans "metabolize" 75 W of this -- convert chemical energy into heat, and use it to do things. Electricity is high "quality" (low entropy) energy. If you have maxwell's demon (the ability to order energy without emitting low-entropy energy such as heat), then there is energy everywhere and using metabolism for it is ridiculous. If not, then metabolism isn't enough energy.
– Yakk
Nov 10, 2019 at 19:30
• I think you may be trying to over-analyze magic. Reminder: it's called "magic" for a reason. I'm perfectly happy stipulating that the function of this magic is to convert metabolic energy into Work, as well as limiting the precision of Work that can be performed. In particular, I'm pretty sure it can't manipulate anything below the molecular level, if that; at least, not with any more precision that heating/cooling stuff. Nov 10, 2019 at 19:45
• I've been thinking about this more, and an obvious "solution" is that magic can only redistribute entropy, not destroy it. IOW, the "energy cost" of whatever entropy is "destroyed" must be payed in addition to the energy just to do the Work. But how does one measure this cost? Nov 11, 2019 at 21:17
• @Matthew So, what we colloquially call "energy use" is almost entirely "entropy cost". The heat we emit when we work? That is energy that is "burned"; converted to a high-entropy state. There is efficiency; what percentage of the energy we convert from form A to form B is not emitted as waste heat. So an internal combustion engine is about 20% efficient: 80% of the chemical energy is emitted as waste heat, 20% is turned into useful, controlled kinetic energy.
– Yakk
Nov 11, 2019 at 21:27

I'm adding this for posterity as the answer I'm actually using. Although I arrived at it independently, Nolo had basically the same idea first, and gets the "accepted answer" credit.