The catenary poles behave like rail signals for the placement restriction (only next to rails), but they're turned into actual power poles as soon as they're placed. Those power poles are then also connected to each other and to your main power network, so there's no extra disconnected electric networks either (as those also eat UPS).
The actual UPS concern would be from the locomotives, because they need Lua scripting to check if they're powered in real-time. That said, i've optimized the heck out of the code, using the minimum amount of API calls and bucketing updates over multiple ticks, so even their performance impact is quite low.
