Lee Carré (2021-07-02 01:26:06+00:00): “I'm not an expert on power mapping”
I'm certainly not, either.

I'm moderately familiar with electrics (besides general interest, for example I was an AV tech in the past which exposed me to memorable things such as 3-phase 125A cabling (notably how heavy it is, even in short runs) with enormous Ceeform connectors on the ends), but utility network level is beyond my experience. But, gotta start somewhere.

Anyway.

“might power=transformer fit even better for small installations?”
I also duly consulted the wiki, having the same question. I think it was on the page about transformers where I read an FAQ about this very matter.

The conclusion was that power=transformer is for an individual piece of equipment (compare an amenity PoI within a building). While power=substation is for the general location (area, building, whatever) which contains (perhaps multiple pieces of) equipment.

Aside from that, there's also changeset #105096082 in which another (experienced) mapper opted for power=substation from a note a had added earlier. So, I followed his example.

Some of the locations host multiple pieces of equipment (I assume transformers, from their appearance).

One of the recurring snags, is that while the signage might be obvious, the actual electron-plumbing is typically either inside a (small) building or behind a completely opaque (and tall) gate.
I would also be guessing that they're transformers (since I don't KNOW that they are). There's other varieties of network equipment, too. An obvious one would be switches (which I imagine the larger substations very likely have). Thus, substation is a safe somewhat-generic valve for the power key. That way, locations are being marked for easier re-surveying (of specific equipment, or when in a dedicated building then marking the outline as a way tagged with building=service and the various power=* related tags, instead of just a PoI node) when detail can be added. Basics first, essentially.

“maybe a "substation=*" tag could be added if you find out what kind of substation these are (minor_distribution?)”
That's been a nagging issue; I don't know what type they are, and suspect that one would either need the knowledge of a utility engineer or to ask the operator.

Thus, I very deliberately left that field blank. Same goes for voltage. Ironically some street_cabinets have rating plates on them, yet substations don't.

Besides, not all are the same. Just the other day I happened upon one (node #8880775659) which actually stated (on a separate (old-looking) sign to the ‘electrocution awaits behind this door for anyone determined to open it’ one) that it was 11kV (from memory (which isn't to be trusted in my case) JEC state that they offer 240V, (something like) 400V and (I gather for industrial customers who need it) 11,000V). So, was pleasantly surprised to find such an 11kV substation. Yet, I can't think of any industry near there (it's all retail (or comparable commerce) & residential around there). So, I suspect that it may be a more major substation (rather than, like you say, just a strategically-placed step-down transformer). There were suggestions from the location of the door and geometry of the building that it was a larger-than-average substation too.

I really would like to be able to add all the related (electrical properties) tags, but I would be blindly guessing. So leaving unspecified seems best (to avoid misleading).
Even if I were an expert, and could determine from a casual inspection, that won't be verifiable to other surveyors (unlike if it were printed on signage, as in the one case I mentioned re voltage).

To me, accuracy is more important than completeness.

However, in future, despite my doubts, maybe such info can be extracted from the operator (even if that means avoiding official channels, and finding a cooperative engineer who either knows or can find out).

A major part of why I mark them when I find them during a survey outing, is that based on experience of the cultural attitude here, I highly doubt that even a list of locations (let alone operational details) would simply be provided upon request (I can foresee the ‘security’ catch-all excuse being readily cited). So, I haven't even bothered asking. They can't stop discovery-by-survey, though. Survey also avoids any copyright problems (since, to my (non-lawyer, but well-read) comprehension of copyright, it counts as my own work).

It's considerable work (though, I'm capturing other data along the way, but thus I also appreciate your recognition of it), but seems more durable than copying from someone else's list.

I say that while also being of the opinion that such data should be public-by-default (including being actually published-by-default, too). But, it is my learning about copyright that makes me wary of the risks. Data from survey is most definitely the belonging of OSM (which I value, being anti-proprietary).