O.P. said:

Quote:

"The problem is this: The standard bag I read about PCT thru-hikers carrying is 20 degrees. ...
I don't really know what temperature bag to get...
Will I cook in a 15 degree bag? How do I tell if I'm a cold or warm sleeper (I tend to get cold)
Will my long underwear be the key..."

And later asked
As I think has been mentioned, metabolisms vary a lot, and your needs might even change as you lose potentially a lot of body fat on a thru-hike. What worked well for me was to start the hike with a 20F rated bag, and switch after I got through the Sierras to a 32F rated bag. I finished early enough that it wasn't getting really cold yet in northern WA; a 32F bag might be skimpy for the last couple of weeks depending on the particular year and how late you finish but was fine for me (good thing, as my wife hiked the last stretch with me and she definitely wanted the 20F bag!).
And of course as has been said here already, bag ratings vary a lot. I used WM bags, an Ultralite to start and then a Summerlite. And btw, relating to another post, I've had both of these refilled, and I did ask for them to be overstuffed at that time; I think this is probably the most effective payback for weight carried --- a little more down in the bag, as much as the manufacturer feels will still loft well.

I'm not saying that a 15F bag would be wrong for you. I don't know, and neither does anyone else here; insufficient data. If you have access to a decent 20F bag you can borrow, find somewhere this winter that likely gets down to lower 30's, upper 20's. For my part (my experience, my year), the coldest I think it got might have been mid-20's and almost always warmer than that. I had (always have) some puffy clothing that I can wear inside a bag that is large enough to allow that clothing to loft, i.e., I can augment bag warmth that way as needed. It wasn't often needed on the PCT for me.

In terms of whether you might be too warm, I do suggest that you select a bag with a full-length zipper, so that you can open it up fully to act as a blanket.

Quilt, bivy combo: I've never used a quilt (other than, again, opening up a bag) but I'm not much of a fan of bivy's. Bivy is for what exactly? If rainproof, it's pretty heavy. If not, then you need some other sort of shelter. If it's to keep bugs off of you, it can be too warm to want to be in the bivy yet the bugs are still swarming. It can get you and your bag wet from sweat. I carried a very light (not water *proof*) bivy for the first 700 miles to combine with a poncho. I used that bivy maybe twice, the only times that bugs were any sort of issue. I'd go with a light tent, unless you're not bothered much by tons of bugs, in which case a tarp and headnet. Or the hybrid approach I used (poncho as rain gear and shelter for first 700 miles, skip the bivy, switch to a single-walled or other light tent at Kennedy Meadows).

I'd also point out that some tents will add a bit of warmth (if you get a good light one, however, don't count on much of that). The selection of a bag is thus dependent not just on your metabolism and personal sense of what's too cold, but also based on clothing you can (and are willing) to wear inside the bag and on your shelter choice. And, heck, your experience at site selection. And probably other things I'm not thinking of.

Go out and do some relatively cold weather hiking and find out for yourself; if you don't already have some, it would be good for you to get some experience at hiking and camping in snow anyway! To be clear, I don't think I ever camped right on snow on that trip, though came close a couple of times (in the Sierras only). But you'll likely hike through a lot of it, and being comfortable/safe in cold/wet/snow is a pretty helpful skill before going into the Sierras.