Whether a 20-degree bag will be too hot depends on the outside temperature - and I don't mean that as a wise-guy response.I'm assuming that your question is, "If I carry this year-round (as my general purpose sleeping bag), will it be too hot in the summer?"

It depends on what your summer weather is like. If you're in the lower Midwest, yeah, it probably will be. If you're in the upper Midwest, or in the New England mountains, it may not be (I don't have the experience there to know.) For a lot of years, I carried a 20-degree bag in the lower Midwest as my only bag, and it worked. What I found was that if it was hot out, I could just leave it in my pack and sleep with no covers. If it got chilly toward morning, I simply used it unzipped as a quilt, draped loosely over my shoulders and upper body, or maybe with only my feet in it, or whatever felt good - kind of like sleeping in my bed at home with the window open. In 40 degree weather, I'd sleep inside it, but might only zip it to my waist; as it dropped toward 30, I might zip up, but not do up the hood or tighten it around my shoulders.

In the southern US, I'd expect (based on a few mid-summer camping trips to Virginia and backyard campouts in August with granddaughters in Georgia), it may very well be too much bag. Down there, a light fleece blanket is probably plenty unless you're going up into the mountains.

In North Carolina, I'd guess the answer is, it depends on where you go. If you head north or northwest, the bag will probably work fine; if you head south, it's going to be excess weight.

But for starting out now, you should worry more about the lows you expect to encounter; you can always not use the bag when it's hot, but you can't make it warmer when it's too cold. If you regularly find, later on, that the 20-degree bag is overkill in the summer, you can always add a 40 or 50 degree bag to your collection later (REI carries, I think, some cheap light bags advertised as "travel sacks" or "travel bags." You could also see if a "liner" would work (and use it to extend your bag's rating in colder temperatures.)

I hope that helps; I really don't have a frame of reference as to how general-purpose a 20-degree bag is in the western mountains or other areas.