Almost certainly your computer. Your monitor alone consumes 65 watts, and your computer around 100 watts at idle, perhaps up to 200 if you are doing something processor-intensive (watching a movie or Youtube video, for instance).

An average human puts out about 100 watts, so your computer has you beat by almost a factor of 2, and maybe more.

Well we could do the math. Pretty much 2000 Calories per day is average so that works out to

2000/24hrs /3600 = .023 calories per second. I went on to an online converter to convert that to joules and got 96.3 joules per second which is watts. So you use about 96~100 watts. My computer uses around 250~300 Watts so my computer uses more.

Of course, efficient computers like laptops only consume a few tens of watts. And if you’re exercising you’ll consume an extra few hundred watts, maybe even a kilowatt for brief periods of high exertion. A top-class athlete can output 2 kw for a short duration, and that’s at something significantly less than 100% efficiency. ETA: Human muscle efficiency is the ballpark of 20-25%. So your world-class athlete could be consuming around 10 kw for short periods of time.

So if you’re Lance Armstrong, using a laptop on a stationary bike, the result could be totally reversed. But Absolute is right for normal situations.

Yes, unless you’re storing some of that energy somehow, maybe by lifting heavy objects. But when those heavy objects fall down to where they were originally sitting their potential energy becomes heat again.

I don’t understand that. If you move your arm, it takes a certain amount of energy, right? That energy is converted to movement. I thought only wasted energy becomes heat.
Help me out here.

Right, so you temporarily convert chemical energy in your muscles into kinetic energy. But what happens to the kinetic energy when your arm stops? It’s lost to friction, impact, etc, which ultimately becomes heat. Everything turns into heat eventually.

As a general rule, whenever the question is “where does the energy go?”, the answer is “heat”.

For comparison, 1 horsepower is about 750 watts, so a human athlete can put out about two and a half horsepower (and not even just for a short time; Lance Armstrong (if you consider him human) can keep up something close to that rate for hours on end). What I get from this is that the standard horse is a slacker.