as others have noted, hills take a lot more power than flat ground, especially if you are heavy.
the mistake most people make is going too fast. then they implode and suffer up the hill at a snail's pace, or stop and go back a different way. most people have problems with perceived exertion and also lack power meters, so they don't know that they are going way too hard until it's too late, and they pop.
it depends on what kind of hill we're talking about, mainly how steep and how long you will be climbing it. for a short hill that you spend less than 2 minutes climbing obviously you can dump a whole bunch more power into it than you can afford to do if you're climbing for 30 minutes.
if you don't have a power meter, you might get some use out of riding with a buddy who does have one. the power number itself won't help you, but you can watch them hold 150, 250, or whatever number of watts when transitioning from flat to climbing and watch the speed plummet.