Stranger Things has had a long-running motif of lights and electricity, but how does the Upside Down affect it so much? The motif has become so well-known amongst fans that it's been taken as common knowledge in the show, something that just happens. The explanation for why this is, though, can be overlooked.
The creepy technology interferences began in the Byers' home with some mysterious cold calls. Joyce could hear intense breathing through the receiver until the phone would shoot electric sparks and fry itself, even leaving scorch marks. Lights and electricity have come up again in Stranger Things 4, Vol. 1. When the older group are in the Creel house in the Upside Down, they begin to talk to Dustin, Lucas, and Erica through the lights. Nancy comes to this resolution by ing Will speaking to Joyce in season 1 through Christmas lights. But how does the Upside Down affect the electricity to such an extent?
It is partially explained in the season 1 episode entitled 'The Flea and the Acrobat' by science teacher Mr. Clarke. Dustin, Lucas and Mike talk to him about how to, ''theoretically,'' travel to alternate dimensions. Mr. Clarke tells them that it would take an enormous amount of energy to tear open a hole (or a gate) in the fabric of time and space to get to another dimension. Then, if it were open, it would disrupt both gravity and the magnetic field, amongst other things. This is because the alternate dimension in question is basically folded in on its mirror image dimension. It could be that this tear in time and space resulted in a massive amount of dormant energy in the Upside Down, which could be the reason for electrical disruption. This could then increase magnetization and the reach of electrical fields, which, in turn, would affect its sister dimension; the Right Side Up.
Because they are mirrors of each other, the infrastructure in the Upside Down seems to be the same as the Right Side Up. The arcade sign in the Upside Down still lights up in season 2, as does the Starcourt Mall entrance in season 3. In season 2, a worker in the Hawkins Lab is sent to the Upside Down to fix a power box when their equipment goes down. When this is fixed in the Upside Down, the power comes back on in the Right Side Up. This further confirms not only that the electricity in each dimension feeds off of each other in a way, but that the electrical field of the mythological Upside Down has a far reach.
Also, due to the immense energy in the Upside Down, it could be that, upon entering it, people themselves become electrically charged to a certain extent, or they become magnetized, which allows them to affect electricity. In Stranger Things season 1, Joyce and Hopper activate hallway lights in the Byer house of the Right Side Up merely by walking past them. Then, if lights are already on, getting closer to them can make them glow brighter. When the Demogorgon breaks through into the Right Side Up, the lights got frantic and sometimes blew, perhaps because the monster has been charged with this energy for a long period of time whilst it has lived in the Upside Down.
At certain points in Stranger Things, the characters can also hear each other multidimensionally. Jonathon hears Nancy cry for help in Stranger Things season 2, and the older group in the Upside Down can hear Dustin in season 4. The two dimensions, then, are already very thinly veiled, so it seems fairly sensical that a huge system like electrical infrastructure would also show some signs of being affected in general. The physics of the show has been fairly vague and not extensively explained, but perhaps some concrete answers will come as Stranger Things heads into its final season.