Solarpunk technology

Agreed about the appropriate technology for the job. That's one of the reasons I think it's very much worth considering all sources of power, but also reconsidering some ways we've industrialized around profit motives and while ignoring externalities. A lot of technologies were in use recently (last 100 years) that might be a better fit for a more solarpunk world, but were dropped because they weren't as fast at making product, or because modern power or fuel are so cheap. I'm currently studying up on metal smelting and concrete production to see which designs seem easiest to adapt to a to a society that prioritizes minimizing externalities (like pollution, greenhouse gas, habitat destruction) over generating as much wealth as possible.

And I think there are some cool old designs with potential (and, as always, tradeoffs). For example, in all the scenes I've done and have planned, you'll see cable-powered streetcars and trains, rather than battery-powered electric busses. I'm not against batteries by any means, but they're a limited resource. Streetcars worked fine for decades long before batteries were anywhere near efficient enough to move a vehicle, and having the cars powered directly by grid the means more batteries available for other tasks, or simply less need to destroy habitats mining for the materials to make the kind of maximum-efficient batteries needed for onboard vehicles (and fewer to recycle after they've been used and reused long past the end of their functional life).

I think analog technologies, like water wheels and solar furnaces and solar cookers might also fit this niche. One of the things I really like about the solar furnace designs, is that they're so simple. Mirrors, framework, and established formulas for overall shape, and you can produce incredible heat - up to thousands of degrees. The materials are commonly available, and require very little tech base to produce or assemble, and they can take some of the highest-resource-consuming tasks off the grid. They're not as reliable as electric power (and electric power from solar isn't as reliable as from fossil fuels yet) and that's a trade-off, but the right combination of technologies, and some adjustment of expectations and schedules, could significantly drop the overall requirements for the collection and storage and distribution of electricity. Water wheels might be also be able to lighten the load from industry/manufacturing on the grid.

I should probably have made it clear that this scene, if I do it, definitely isn't a factory. It's a workshop, and they tend to be much more generalized spaces. I've spent pretty much my whole life in workshops, and I've never been inside a factory, so I won't be trying to depict a solarpunk assembly line, though I do believe we'll need them. I will say that for the spaces I've been, from personal workshops to makerspaces, to teaching classrooms, it's very rare to find more than one of each large machine, usually both for cost and physical space limitations. This isn't as efficient as a factory line with each person doing one task, but cottage industries can be incredibly flexible and powerful. Just look at the production of military equipment in homes and garages in almost every country in WWII, or if you want a more recent and positive example, the way the 3d printing community stepped in and provided an instant-turnaround communal mass production of face shields for medical professionals during COVID. In the first few weeks, while industry was talking about how long it would take to re-tool and step up production to meet demand, regular people churned out thousands of the things and filled that gap. And because the makers were themselves distributed, physically getting the goods to the people who needed them took less travel, mostly using volunteer distribution networks.

As for useful work, one of the cool things about the death ray is that once you've got the structure built/modified, and the focal point established, it can do all kinds of jobs. So far I've seen Fresnel lenses burn through steel, melt aluminum, and sinter sand into glass, and that's without the mirrors collecting additional light. With the right setup, you could basically roll/drag different tools into the workspace depending on how you wanted to use the beam of sunlight. With computer control of the mirrors and shutters, you could dial in how much power you want, or regulate against inconsistent light if you need less than max and clouds are passing overhead. In addition to the tasks I mentioned above, you could use it to heat metal quickly to bend it like a forge, or use it for other kinds of crucibles/materials, if you can dial in the temperature they require. And if nobody has any projects cued up for the day, you could roll a steam-boiler-based generator under it (likely you'd want this to be taller than the focal point so the light would be better distributed and less likely to overheat a specific spot) and charge the facilities' batteries.