Likely due to the efforts of the lab
It’s quite likely that the DoE lab
is ensuring that the infrastructure is maintained. In the second episode, they send a soldier in to fix something:
DOCTOR OWENS: How’s it looking out there, cowboy?
SOLDIER: Uh, you know, Doc. The usual. Nice and nasty with a chance of
radioactivity. Oh, yeah, it’s barbecued all right. Anyone hungry?
SCIENTIST: And we’re back on!
Stranger Things, “Trick or Treat, Freak”
And then a bunch of indicator lights go on.
The thing they’re fixing definitely seems to be some sort of power box.
The clear implication is that the lab has some power infrastructure set up. It had gone out, which means that it was running before. It was likely set up after Owens took over (which is why, I think, we didn’t see any lights there last season).
Presumably they’re taking advantage of pre-existing infrastructure in the Upside-Down, not running power in. As to why that infrastructure exists in the first place, it’s uncertain, but see my theory.