Wizards built a civilization on a barren planet.
More details here.
The population of the planet is between two and four million wizards, but not all of them are born equal. More than half of the population don't produce enough magic, or lack the focus, to actually cast spells.
They still produce some magic, that can be harvested and stored in magic batteries. The collected energy can be used to power things like sentient automatons/robots, flying boats, atmospheric spells, etc.
To charge a battery, people have to carry it around with them, see illustration bellow (pardon my poor drawing skills).
Depending on the person, a battery can take from a few hours to a couple of month to charge. Once fully charged, there's no difference between them, no matter who charged it. If not damaged, batteries can be used and recharged indefinitely.
The battery market
I still don't know how to regulate de battery market.
My goal is to have most low-level wizard walking around with a battery strapped to their back.
I'd also prefer this society to not be totalitarian.
Because of the context, I already have troubles keeping the government from controlling too many aspects of its citizens lives. It's already in charge of the maintenance of city-sized atmospheres and manages the terraforming process. Here the worst-case scenario would be the government forcing people to charge batteries.
So here are a few possible ways I found to organize the battery market :
Batteries are cheap
Empty batteries are so easy to produce, even poor people can afford to buy their own and sell it to whoever they want once full.
Advantages : Free market, next to no involvement from the government. Most low-level wizards would choose to carry one, since it would be an additional income for very little work.
Problem : This society lives on a not-fully-terraformed planet, and I'm afraid any kind of good or resource being abundant and cheap would sound weird.
People don't own the batteries they're carrying
Batteries are owned by businesses, rich families, or public institutions. They pay people to charge their batteries.
Advantages : Sounds maybe a little more realistic than the previous option. Low involvement from the government.
Problem : This would be an oligopsony (few buyers deciding the prices), leading to lower market prices. This could deter some people (who have an average-paying job, or who would take too much time to charge one battery) to enter the market.
The first battery is free
When they turn sixteen, every citizen gets a free empty battery from the government, and when they sell a charged battery, buyers are required to give them an empty one, in addition of the payment. Like for the previous options, people aren't forced to do it, but choose to if they want the additional income.
Advantage : Resolves the problems of the two previous options.
Problem : Heavy government involvement (even if it doesn't decide the prices or forces people to wear batteries).
Knowing all that, how should the magic battery market be regulated?
(Feel free to build from one of my solutions or to give an entirely new one)