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The Sidereal Record Straightener
Posted by muriel_volestrangler in Latest Breaking News
Tue Aug 11th 2009, 04:58 PM
In 2000, a formula for 'mpg equivalent' for all-electric vehicles was published.

http://www.epa.gov/EPA-IMPACT/2000/June/Da...

The end result of this is to find the 'petroleum-equivalency factor' (PEF). There are 2 values:

PEF = 82,049 Wh/gal (if no petroleum-powered accessories are installed)
PEF = 73,844 Wh/gal (if any petroleum-powered accessories are
installed)

Now, the Volt uses, from a charged battery, 25 kWh per 100 miles.

So, 25 kWh/100 miles = 25,000 Wh/100 miles = 250 Wh/mile.

So the 'all-electric' mile/gallon figure is either
82,049 / 250 = 328 mpg , or
73,844 / 250 = 295 mpg , depending on if you allow for the accessories or not.

They have combined one of those with the 50 mpg the car is said to get when its using power from the gasoline engine, and come up with an overall mpg of 230.

Now, there's another calculation you could make: the amount of carbon dioxide produced from 'standard' grid electricity.

From http://www.epa.gov/cleanenergy/energy-reso... , the average non-base load electricity generated in the USA produces 718g of CO2 per kWh. So that's 718g per 4 miles, or 179.5 grammes/mile. Compare that with the car when running at 50 mpg on the gasoline engine: 8810g per (US) gallon of gasoline, ie 50 miles, = 176.2 grammes/mile.

So if you just plug it into any old socket and use the 'average' electricity generation mix, you won't actually be producing any less CO2 than if you drive it on the gas engine. You have to use electricity generated with a low carbon method. There is cheaper electricity available at night, when demand is low; if that's an indication that they'd really be throwing it away otherwise, then you might say using that would help save carbon dioxide. I don't know if the electricity really is 'thrown away' then, though.
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