How does your power output in climbing the stairs compare to the power output of a 100-watt light bulb? If your power could have been harnessed and the energy converted to electricity, how many 100-watt bulbs could you have kept burning during your climb?

- - My mass is roughly, let's say, 91.8 kilograms.

- - The 2nd floor of my house is, say, 5 meters high.

So, when I lift myself from the first to the 2nd floor, I do enough work to increase my gravitational potential energy by

PE = (mass) x (gravity) x (height)

PE = (91.8 kg) x (9.8 m/s²) x (5 m)

PE = 4,500 Joules

It takes me, say, 10 seconds to make the climb, so the power is

P = (work done) / (time to do the work)

P = (4,500 Joules) / (10 seconds)

P = 450 watts (about 0.6 Horsepower)

This is enough power to keep four 100-W light bulbs and one 50-watter glowing at normal brightness.

But we have some work to do with the direction of the energy transfer.

When I stand at the bottom of the stairs and look up at the 2nd floor, I have a choice: I can either climb the stairs to the 2nd floor, OR I can pedal my generator-bike and light some lightbulbs. NOT BOTH.

I have energy in my blood and my muscles, which I can use to do work:

- - If I use the energy to lift my mass to the 2nd floor, then the energy goes into the mass and becomes gravitational potential energy.

- - If I convert my muscle energy somehow to electrical energy and light some bulbs with it, then it goes to the bulbs and nowhere else. I can't BOTH lift mass, give it potential energy, AND light bulbs.

I can use my energy to climb stairs, OR chop wood, OR pump water, OR climb a ladder, OR turn an electrical generator, OR play with my dogs, OR run around the block, OR let my brain use the energy to solve some math problems. Or I can distribute my energy, some here and some there, to do many things.

But I CAN'T use the SAME energy to do more than one thing.