Normally batteries are sealed systems, consisting of something, typically liquid, that stores electrons (electrolyte) and something that keeps the electrolyte in place and allows only the electrons to go in/out (electrodes).

The amount of electrolyte you have determines how much energy you can store, and the area of the electrolyte determines how quickly you can move those electrons in/out.

The coin cell in your watch is an electrode/electrolyte sandwich. AA-style batteries are a bunch of coin cells stacked together. Car batteries are a bunch of AA-style batteries stacked together. This is done for a myriad of reasons dealing with the safety and efficacy of how batteries work, beyond the scope of an ELI5.

Since all of these batteries use the same coin cell base unit, as batteries get bigger and bigger the ratio of electrode to electrolyte stays fixed, so if you ever want to store more energy, even if you don’t care about how quickly it charges/discharges, you have to have more electrode material, which is heavy, expensive, etc.

To fix this, people started making flow batteries, which consist of a smaller assembly that resembles a typical battery electrode/electrolyte sandwich, but now with tanks to hold extra electrolyte liquid. This liquid is pumped in as needed, and that allows you to store a lot more energy in your electrolyte without spending big on extra electrodes. It also has other cool properties like longer storage times without leaking as much power (self discharge) as a traditional sealed battery because now you can fully isolate the electrolyte with say a couple of valves.

Since flow batteries are designed for more energy storage with lower power output, it was ironic that the article math worked out to the power equivalent of the sun’s impact on Earth.

Lengthy explainer but hope it helps!