A watt is one joule per second. So if a phone consumes 100 watts, and its battery has 1000 Joules, it'll run for ten seconds. If its battery has 8 megajoules (which is as much as an average person eats in a day), it'll run for eight thousand seconds. Which is about 2 hours. It's easy and simple.

I understand watts. I don't really understand amps. I know how many watts My PC draws because I installed the power supply Myself. I don't know how many amps it draws. I don't know how many amps My phone draws, or My wireless mouse, or My body.

My phone's battery lasts longer when I turn the screen brightness down. Does that mean the screen is drawing a variable number of amps depending on the brightness? That's confusing! I don't really understand what an amp is, so I can't visualise that. I can understand the fact that a dark screen draws less watts easy, because watts are just energy per second.