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).
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.)
- 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 😉.)
- "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 — especially wearable stuff — 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.