Hey everyone!

I had an idea for a town in my campaign that harvests raw magical energy and sells in in the form of "batteries," which would basically function as wands with spell slots that aren't limited to any specific spell. Wealthier people could buy these batteries for domestic purposes.

What I'm wondering is how something like this would scale in terms of price and level. If these "batteries" can be used for anything, how much energy (spell slots) would a cantrip expend versus a higher level spell? Is a level two spell double the price of a level one spell? Is a level one spell double the price of a cantrip?

I've been playing d&d for several years now, but I just recently DMed my very first session and am still trying to get to grips with the more numerical side of things.

Edited heavily: Apparently math is hard :p

 I'm just spit-balling here, but I notice that the XP needed to unlock the ability to cast a spell level doesn't go up lineally, but as a curve.

You need to be level 3 to cast 2nd level spells ( 900 XP ) and level 5 to cast 3rd level spells ( 6,500 XP ), so it take 7x the amount of education/experience to be able to unlock the next level of magic up. And it gets worse, the higher the level of spell. Spell levels unlock roughly once every 3 Character levels, but required XP to go up those levels goes up on this curve:

If you view the required experience/education as a gauge for the complexity and energy requirements of the spell ( and that's something you may, or may not, agree with ), and extrapolate out, then a 9th level spell would have 250x as much power as a 2nd level spell.

I would think it very unlikely that you'd see "spell batteries" capable of fueling anything higher than maybe a 3rd level spell ( and I think those would be very powerful and expensive, given that fireball is 3rd level ).

Also - consider that you're essentially introducing the magical equivalent of the Industrial Revolution to your campaign world. There's absolutely nothing wrong with that - many people have played around with elements of Steampunk in a D&D High Fantasy setting before - just be aware that's what you're doing. It will help you sketch out the impacts of such a technological development on the cultures of your world.

Yeah I'm totally cool with introducing a sort of industrial revolution! My world is a bit pastiche in terms of tech timelines, where they already have steam engine trains but no guns, so I'm totally fine with mixing this in. I also imagine magic would be a lot less money-efficient than more conventional means in terms of domestic chores, so it probably wouldn't be terribly common outside of the city where the magic can be "mined".  

This chart will be very handy for me, thank you! Where do you think cantrips place on the exp curve scale? I assume this aligns with full-casters like wizards, right?

The math kind of breaks down as Cantrip and Level 1 spells are both available at 0 XP ( so, technically it takes infinitely more XP for 2nd level spells ).

Working backwards, we can see that between levels 2-4 the amount of XP triples, and then flattens out a bit ( proportionally ) for level 5, but still stays ( very roughly ) triple that of level 4.

If we make the ( wildly arbitrary ) assumption that Cantrips are the domain of a 0th Level Wizard  or Apprentice Wizards ( yes, I am just making s^#@ up now ... ) , then I might peg a Cantrip at 1/3 of the power of a Level 1 spell. 

Actually that graph is the XP requirements for general Character leveling.

Below is the Character levels needed to unlock spell levels for full casters

Hmm...This is getting more confusing than I expected, as I look into it. The EXP chart is compelling, but I'm not sure it would be accurate for this case, considering the character's exp is "spent" on more than just spell slots...you know? Like a level five wizard isn't using 100% of their experience to learn fireball, it's also contributing to their proficiency bonus and HP and such.

According to this, a level one spell "should" have 2.5x the damage output as a cantrip, so maybe a cantrip takes 2/5 the "power" of a level one spell? The jump from cantrip to lv 1 is waay higher than the jump between any other levels though. On average it looks like a given spell is 130% the power of a spell one level lower, with the smallest jump being 109% and the biggest being 160%...oof. This is not very clean.

Maybe I'll have the player roll 1d4/spell level to see how many slots of their "battery" are used? And a cantrip could be just a flat 1? That gamut might be a bit wide...

The problem here is that you're trying to map a gaming system to a consistent pseudo-reality - and that doesn't always work :)

Spells do not consistently scale in their effects or the amount of energy required for those effects, as their spell level goes up - or even with other spells within their level. Both Fireball and Lightning Bolt are over-powered for their level, but are what they are because they're traditionally that level and power from older editions ( I've actually done some diving down this rabbit hole, looking at consistent custom "point buy" spell creation systems ).

Any detailed home-brew system I've seen for creating spells ( like the one you reference ), either has problems with internal consistency, or cannot recreate the spells in the PHB/DMG because  the published spells are not themselves consistent. 

The "in game" rationale here is that those spells are common because they're extraordinary works of genius efficiency and magical effects from the wonder-wizards that created them long ago, but most modern run-of-the-mill Wizards couldn't recreate spells that efficient at that level - sort of like I don't expect modern composers to produce music that's as tight, compact, and elegant as a Bach fugue ( although it's still possible - you get a genius every now and then ), and how Bach's works are widely known and performed.

Ultimately, we may be over-thinking this. You can just stick with your original geometric '2x the power of the spell level below it' ( or whatever conversion factor ) if you like, as it sounds like this technology is restricted to a given region, for some reason ( magic is not mine-able elsewhere? The storage crystals can only be found here? ), so you can allow the technology to be somewhat disruptive and inconsistent with the use of magic elsewhere, and not set your entire campaign world ( or campaign ) on its head. 

The end result might not change that much: spell batteries able to power higher than 3rd level effects are probably unheard of, and 3rd level batteries would be super-expensive and rare; on par with a Hospital's entire backup power generation system that kicks in if their grid connections go down.

Are you just trying to make a rod of absorption or something more like a mizzium apparatus or like a combination of the two?

I like this idea, but I also like the idea of avoiding tons of math. (I took a chess class in college for a credit of math to avoid doing real math.)

My suggestion is to keep it simple.

• You can store X number of levels of spell magic in a battery (X will depend on factors you choose with below considerations, but also maybe skill of battery maker and materials used...so you can have cheap and possibly risky batteries, or good ones).
• You can use a max of 3 levels at any one time (regardless of X in the battery). There's just a limit to how much energy the batteries can output. This avoids someone dropping 9th level nukes all the time.
• Thus, at any time, you can spend 1, 2, or 3 levels from the battery to cast a 1st, 2nd, or 3rd level spell.
• Cantrips: You get 3 cantrip casts from 1 level of the battery. Why 3? Because it keeps the batteries from turning non-casters into Eldritch Blast slaughterhouses. And if people complain it's too much power for a cantrip, the explanation is that there's also a minimum amount of energy a battery will give out at any one time. 

Done and done. Easy to keep track of, with only cantrips needing you to track 3rds of spell levels. And going this route gives you the following plot points:

• Frikking Wild West. If these can be used by anyone, anyway they want...that guy over there slept with your spouse? Fire Bolts all up in that tavern, cast by the angry butcher with the battery.
• Someone is trying, or has, figured out how to release more than 3 levels at once.
• Some batteries are poorly and cheaply made. Wild Magic Table!
• What happens when you toss a battery at someone and then shoot it with a magic missile? Boom?
• Someone is corrupting the magic in some of the batteries. Someone is tainting the magic, and using those batteries is turning people evil, out of control, psychotic, etc. Suddenly there's a huge influx of very inexpensive but very well made batteries, with tons of power. Where are they coming from? Everyone loves them. But every so often--someone goes nuts and blows up their whole family with no warning. Who's making these!?