This is definitely a simplification, which is why I pointed out the possibility of distributing costs among the consumers based on how much of the total consumption each consumer is responsible for.

I think the major point still stands though: In order to take advantage of production at scale, you need to build some minimal size production facility. For stuff like hydropower, that minimum can be quite high, depending on available geography.

If marginal cost is zero, it makes most sense to charge some form of flat rate to have access to power, rather than a consumption-based price, because it’s not necessarily feasible to downscale the facility, even if there’s low demand (in that sense, hydro or nuclear would be better examples than solar).

The details of how this more or less flat rate should be distributed among consumers is a discussion in itself (should those living further away pay more since they require more power lines? etc.)