The downstairs probably just generated in a wall somewhere. Press F9 to see the entire level.
If by some chance it
really didn't generate, then you can use noclip down (you have to bind it to a key first) to get there without using a recoiler every time.
Also, I assume you mean grit gate, because thats where the city is, and the rusted archway just goes down to lv 4.
Chem cells are still useful if you have Tinker 1 and can recharge them with a red bit. They last much longer than solar cells, meaning less having to swap them in and out of your inventory. Whether their "keystroke" efficiency makes up for their resource cost is purely for debate; its entirely minor either way.
There are also antimatter cells, though far less common, expensive to tinker up, and I don't know how much charge they have, whether they use the same materials to recharge, or even if you can recharge them at all.
With the possible exception of in recoilers, solar cells have enough energy to last quite a while, and if you have two or three in your inventory, then you should be pretty much be able to power all your stuff indefinitely (excluding super deep dungeons possibly), making them quite a bit more useable then chem cells.
Antimatter cells have a lot of charge, and require 15 red bits to recharge (vs 2 for chemichal, and 12 for nuclear). A solar cell can't even fire a phase cannon once, a chem cell can fire one a single time (with some energy left over), a nuclear cell can fire it 13 times, and a antimatter cell can fire it 26 times.
It also starts out drained after you build them.
So antimatter cells are clearly the most efficient once you build one, and a full charge one would probably last you the entire game if you put it in anything besides a phase cannon.
I've never seen a antimatter or nuclear cell except by wishing, so you will have to find a datadisk and get lucky.
That said, its not like you will ever run out of energy if you have tinker one, since you can just use solar cells, or charge chem cells pretty much forever on the red bits you find.