I don't think you could even have anything approaching 1970s level of tech and be self-sustainable in a small city. Would you know how to make a discrete transistor? How big a community would you need to spin up even a basic electronics factory?

I think 50,000 people means you're not even at the industrial revolution level of tech. You're certainly not going to be able to manufacture any electronics. That completely limits the power generation options. Also, you'd have to use pretty basic materials, because 50,000 people isn't enough to have advanced materials processing facilities. You'd also have a severely restricted diet, growing only whatever the local climate permits. It would probably have to be vegetarian, unless you had enough space for hunting (with bows and spears).