Sunday, April 20, 2008

What is a watt?

Dear Mick:

I'm having a brain cramp, and I need some help. The "Watt" listed on the top of the lightbulb is the Watts-per-Hour the bulb uses right?


Dear XXXX:
Yes. It's a truncation, but a confusing one. Household "watts" are a measure of power flow, like cubic feet measure stream flow for a kayaker. But they are not the same things as System Internationale watts, which are a measure of power. Household watts are actually watt-hours.

Flow is somewhat meaningless without knowing the amount of time flow is sustained, so the kayakers say "cubic feet per second." Likewise, electrical engineers use "watt-hours."
A watt-hour is actualy a formal SI watt sustained for an hour. A thousand watts is actually a thousand watt hours, or one kilowatt-hour, or 1 KWH, or 1000 SI watts sustained for one hour. Your power consumption is measured in KWH by Central Maine Power when they make up your bill.

For household uses, light bulb companies mistakenly rate their products' consumption by watts when they 'watt" they really mean are watt-hours. A 13-watt compact florescent bulb actually uses 13 watt/hours per hour of use.

From wikipedia

"The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second."


"One joule is the work done, or energy expended, by a force of one newton moving one meter along the direction of the force."


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