Here's a good article on EN13537 standards for sleeping bags:
And R values for sleeping pads (no standard testing so we're at the mercy of the manufacturers' marketing departments):
The dummies in the EN13537 tests are dressed in base layer top and bottoms and wear a knit cap. I've read (but didn't bookmark the reference so now don't know where it was) that the tests for 20*F (-7*C) sleeping bags use a pad with an R rating of 5. I note that An-D has also stated that above.
As mentioned there are three ratings (other than the "too hot" (my term) rating, something I'ver never experienced as long as the bag has a full length zipper): the "Comfort" rating, for women/cold sleepers, the "Lower Limit" for men/warm sleepers and the "Extreme," meaning the temperature at which you may not be dead of hypothermia by morning. Watch the ratings carefully, because firms that don't sell their sleeping bags in the EU and therefore don't have to abide by the regulations are liable to confuse the different ratings. This is more of a problem with retailers than with manufacturers. Make sure you (and they) know which rating is being discussed. Look at the manufacturer's website, not the retailer's. It should show all three ratings with at least a graphical explanation.
I personally have found the "Comfort" rating (about 9-10*F higher than the "Lower Limit" rating) about right for me. (I'm female and older than most.) While Western Mountaineering doesn't publish their EN13537 ratings on their website, they do sell their bags in Europe and their EN ratings can be found on UK retailers' websites. For my Ultralite, I found that the "lower limit" rating is 17*F and the "Comfort" rating is 27* F (converted from Celsius and rounded to the nearest whole number--and no, I didn't write down the C number). I have to start adding insulating clothing when the temp drops to 26-27*F, which bears this out. With a sufficiently warm pad, I'm fine at 15*F with all my insulating clothing on. Your mileage, of course, may vary--we all differ in how much cold we can tolerate, as in many other areas.
The dummies are supposed to represent the "typical" man or "typical" woman, whatever that is! Some of the testing procedures have been a bit suspect (different ratings from different labs). However, it's the closest anyone has gotten to a standardized temperature rating system, as opposed to the fictional ratings provided by marketing departments.
Generally, the high end ($$$) bags from firms with an excellent reputation, such as Western Mountaineering and Feathered Friends, are quite accurately and usually conservatively rated. Do note, though, that for those that don't test their bags, their reported temps are probably closer to a "lower limit" rating than to the "comfort" rating.