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Posted by Paul A. Heckert Jun 18, 2008 |
I recently read in an article that vending machines consume 3000 kilowatts per year. The article was about installing devices that turn off the electricity consumption when people aren't around. That sounds like a good idea, but the statement is technically incorrect.
Kilowatts is a unit of power consumption, so the time is already included. So any statement that specifies the amount of time for a watt or a kilowatt must be incorrect.
Energy is measured in joules. Power is the energy divided by the time and is measured in watts. A watt is a joule per second. So a machine, vending or otherwise, that has a power consumption of 3000 kilowatts uses 3000 kilojoules of energy every second. But saying it uses 3000 kilowatts every year makes no sense. It's like saying that a car drives a total distance of 60 miles per hour every year.
To me, 3000 kilowatts sounds high for a vending machine. The main power consumption would be a few light bulbs and a refrigerator to keep the drinks cold.
Perhaps it is 3000 kilowatt hours per year, which when divided by the number of hours in a year gives a power consumption of almost 350 watts, which could power a few light bulbs and a small cooling unit. Multiplying a power unit, kilowatts, by a time unit, hours, gives an energy unit, kilowatt hours. Physicists seldom measure energy in kilowatt hours, but power companies usually do. The total number of kilowatt hours used per year makes sense.
Errors such as this one in science articles make it difficult for technically minded people to figure out the real details of the story.