It can be hard to find an error-free article that involves any use of ratios, units, or simple mathematics (if you can call adding and multiplying numbers, mathematics). The use of numbers seems to cause writers to abandon any attempts at checking, despite the skills involved being high school level or below. I think I’ll start collecting them.
Today’s entry, that all-time classic, confusing power with energy. There’s an article in Wired
today about a device that burns waste restaurant oil (though the writer incorrectly calls it ‘grease’).
“Put 80 gallons of grease into the Vegawatt each week, and its creators promise it will generate about 5 kilowatts of power.”
Only, a gallon of oil is a measure of energy, but a kilowatt is a measure of power. Energy is measured in joules (or any other energy unit, calories, BTUs, whatever). A joule is a watt-seconds, watts times seconds. Watts (power) are joules per second, the amount of work done per time, or the rate of energy conversion. The key being, energy is a scalar, power is a rate.
So does this thing produce 5 kilowatts for the full 168 hours of the week, or some lesser period? The writer doesn’t say and probably doesn’t know. Assuming it produces the 5 kW continuously, it would then do 5 x 168 = 840 kilowatt-hours (kWh) of work per week. 840kWh is worth about $126, with power costing about 15c per kWh. So 80 gallons is supposed to generate $126 worth of electricity, say $1.50 per gallon.
The writer goes on:
At New England electricity rates, the system offsets about $2.50 worth of electricity with each gallon of waste oil poured into it.
So now he’s saying a gallon of waste oil generates 25 kWh of electricity, or $2.50 worth of electricity per gallon. Wait, didn’t he just say $1.50 per gallon?
Vegawatt’s founder and inventor, James Peret, estimates that restaurants purchasing the $22,000 machine will save about $1,000 per month in electricity costs, for a payback time of two years.
OK so they’re claiming $1,000 a month, or 10,000 kWh of electricity, or about 14 kW continuous (hey it might be a 24 hour restaurant…). Depending on which of his numbers you go with ($2.50 or $1.50 of electricity per gallon of oil), that’s between 400 and 660 gallons of oil a month. The only number he gives though is 80 gallons a week or about 330 a month. These numbers are not astonishingly out, only a factor of 2, not bad for general writing.
I just saw this article on the Watt
in Wikipedia which has a section ‘Confusion of watts and watt-hours’. “Power and energy are frequently confused in the media”, it says. No kidding.