I unexpectedly ended up with some chicks a few days ago. (long story lol) By the time I will need to integrate them into my flock it will be very cold. I don't have electricity in my coop. Does anyone have any experience with this?
Regarding the temperature more than the actual integration:
Obviously you start by brooding them indoors with a heat source.
At some point, you take away the heat source but they are still indoors for another week or more. At this point they are acclimated to indoor unheated-temperature, but not yet to outdoor temperature.
Sometime after that, you start exposing them to even cooler temperatures: maybe open a window in the room where the brooder is, or move them to an attached garage that is cooler than the house but warmer than outside, or take them outside for part of each day, or something of the sort. You can also combine things, so after a while they may be spending days in the coop (in a look-no-touch setup so they start getting to know the big chickens) and nights in a warmer place (house or garage, possibly with an open window).
Eventually, you pick a time and put them out full time. It's nice if you can pick a warm stretch in the weather rather than doing it during a cold snap. You will probably go out to check them several times in the first nights and days, but hopefully it will all go well.
I don't have a specific timeline for this, because chicks can grow their feathers at such different rates. At any given point, just think about whether they are ready to go to the next stage yet. Inside a house, you might be able to remove the extra heat before they even finish growing their first set of feathers (maybe when the bodies are feathered but the tops of their heads are still downy.) They definitely should be fully feathered before they start living outside full time with no supplemental heat.