What’s a watt? Get on a bike.

I checked the output of our rooftop solar panels just now, at 11:28 a.m. On a sunny, brisk Denver day (briskness doesn’t affect solar output), our 13 east-facing 220-watt panels were cranking out 2,476 watts, according to the inverter in the garage. Wife’s got the dryer running, but I’d assume we’re still feeding power back into the grid. The washer just went on, which just led me to walk around the house to the sidewalk to check the net meter — indeed, still running backwards.

I’ve injured my foot and can’t run at the moment (plantar fasciitis), so I’ve spent a bit of time on the basement recumbent exercise bike. It tells me how far I’ve gone, my heart rate, and watts. I’m skeptical of the distance bit, which would depend on the bike, the terrain, the wind and so forth. The heart rate and the watt count I trust, though.

A watt, according to the Wikipedia entry, is “the rate at which work is done when an object’s velocity is held constant at one meter per second against constant opposing force of one newton.” It adds the descriptor, “A laborer over the course of an 8-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes,” with attribution to  Marks’ Standard Handbook for Mechanical Engineers 11th Edition. A bright incandescent bulb burns about 75 watts (the dryer upstairs, which given the sun outside probably isn’t strictly necessary, to think of it, is probably humming at 800-1000 watts), so the the laborer reference is a help. But what kind of of labor? Ditch digging? Pedicab services? Carpentry?

Get on a bike, though, and you get some perspective. Mark Cavendish, the acclaimed cyclist sprinter, is said to hit 1,600 watts in brief spurts, and Tour de France riders average about 260 or so over the entire race. So the solar panels up on the roof were, at 11:28 a.m., cranking out the equivalent of 1.54 Mark Cavendishes blowing away the field at the  finish line. It’d be  625,000 Cavendishes at speed to do the equivalent of a 1,000 megawatt (1 gigawatt) coal or nuke plant. But they’re just numbers until you get on your own watt-counting bike.

Anything over 230 watts starts to hurt and is more or less unsustainable for more than 15-20 minutes for someone in decent but not great shape (me). That’s less than a tenth of what our inert-looking rooftop-panel array is doing right now. Which is to say: when the sun’s overhead, each solar panel on my roof can do the work of me at stress.

1 Comment

  • Andy Logan Posted April 11, 2011 9:10 am

    Nice blog and a good perspective on watts. I always say that people would be more careful with what they eat if they banked calories. For example, get on that exercise bike and see how hard it is to burn 200 calories. Then, see if that can of Coke is really worth it.

Add Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.