Electric Car Mileage for Chevy Volt from Equivalent Carbon Dioxide Pollution

Dennis Silverman

Department of Physics and Astronomy

U. C. Irvine

http://www.physics.uci.edu/~silverma/

August 27, 2009

Updated, October 3, 2010

Electric cars will soon be produced again, and as the mileage estimate of 230 mpg for the Chevy Volt illustrate, they may well not include the CO2 emissions created in the generation of the electricity that the cars use. We estimate the CO2 emission per kWh for the various California utilities using their 2009 Power Content Labels. In doing this we assign efficiencies of 34% to coal and natural gas plants, and a resulting 2.08 lbs of CO2 per kWh from coal plants, and 1.17 pounds of CO2 per kWh from natural gas plants.

Equivalent Miles Per Gallon for the Chevy Volt from US and California Utilities

The Chevy Volt is quoted as using 25 kWh for 100 miles of city driving. This would be 10 kWh for 40 miles of electric only driving or 4 miles per kWh. The Chevy Volt’s range on purely battery electricity is quoted as 40 miles, and its charging capacity is 8 kWh, which would be 5 miles per kWh. There is already a discrepancy, unless the difference is due to a mix of city and highway driving. I will use the 4 miles per kWh for city driving, which is the same as 0.25 kWh per mile.

The standard emissions from burning gasoline are about 20 lbs of CO2 per gallon of gasoline, irrespective of what mileage you are getting. By using the CO2 emissions to generate a kWh from the above webpage, we can convert to equivalent gallons of gasoline used. The table below shows our calculations, rounded to somewhat indicate their accuracy.

Utility	Lbs CO2/kWh	Lbs CO2/mile	Equiv. gallons of gasoline for 15,000 miles	Equivalent mpg
PG&E	0.59	0.15	113	133
SC Edison	0.76	0.19	143	105
Sacramento MUD	0.72	0.18	135	111
SDG&E	0.87	0.22	165	91
LADWP	1.17	0.29	218	69
US Average	1.34	0.34	250	60
Coal Electricity	2.08	0.52	390	38
Natural Gas Elect.	1.17	0.29	220	68

The results for the equivalent CO2 emission for the Chevy Volt are very area or utility dependent, ranging from 38 to 133 mpg on pure electric battery driving. They also are much smaller than the 230 mpg figure cited by General Motors without considering the pollution from electricity generation.

To compare equivalent fuel usage, an average gas driven car at 25 mpg, driving the government standard 15,000 miles per year, would burn 600 gallons of fuel. A Prius at 50 mpg would burn 300 gallons of gas for that distance.

Equivalent miles per gallon for different classes of electric vehicles

The EPRI (Electric Power Research Institute) ranks electric vehicles of various sizes with their typical power needs, which we include in the next table for the US average CO2 case of 1.34 lbs CO2 per kWh (note the Compact Sedan at 0.26 kWh/mile is very close to the Chevy Volt at 0.25 kWh/mile).

Vehicle Class	kWh/mile	Lbs CO2/kWh US Average	Lbs CO2/mile	Equiv. gallons of gas for 15,000 miles	Equivalent mpg
Compact Sedan	0.26	1.34	0.35	260	57
Mid-Size Sedan	0.30	1.34	0.40	300	50
Mid-Size SUV	0.38	1.34	0.51	380	39
Full-Size SUV or Pickup	0.46	1.34	0.62	460	32

For comparison, a full size SUV averaging 15 mpg would use up 1,000 gallons of gas to go 15,000 miles.

Carpooling as a Much Cheaper and Money Saving Alternative

If you take your present typical 25 mpg car and carpool with one other person, you are each then effectively driving a 50 mpg Prius. However, you have saved yourselves the cost of buying two new Priuses at about $25,000 each plus the financing charges or the loss from savings income on the principal . You also can save on company parking expenses or hopefully receive a bonus from your company for carpooling. The cost of gas is also halved per person. At 15,000 miles per year for a 25 mpg car the cost for 600 gallons of gas at $3.00 per gallon in California is $1,800. Splitting this cost results in a savings of $900 per year for each of the two carpoolers. For three people carpooling, the effective mpg in a typical car for each is 75 mpg. This beats the Volt even in the large Los Angeles area and saves each carpooler the cost of buying a maybe $40,000 Volt (or including the $7,500 rebate cost to taxpayers) plus finance charges or interest lost, and gives them each a $1,200 gas savings from their present typical cars. Finally, if four people carpool in a Prius, they achieve the bragging rights of an effective 200 mpg for each, which as we have seen even the Volt cannot attain.

The misperception that miles per gallon matters, compared to its inverse, gallons or pollution per mile.

Since the environmental considerations are really about the CO2 emission per mile driven, the useful figure is really equivalent or real gallons used per mile or per 100 mile or per year at 15,000 miles per year. Here we see the law of diminishing returns. Every supposedly impressive doubling in miles per gallon actually decreases an already small fuel usage by half. Let’s compare the largest sized SUVs or small trucks of past years that only get 12 mpg, and call their unit of pollution as 1, since they are the worst source of pollution. The average present US vehicle at 25 mpg is then ½ in pollution units per mile. The future 2016 standard or present best gas engine cars at 35 mpg is 1/3 in pollution units. The Prius at 51 mpg is then ¼ in pollution units. The local SC Edison charged Chevy Volt at 100 mpg is then 1/8 in pollution units. So for example replacing an average car at ½ by an expensive Volt at 1/8 will save 3/8=0.38 pollution units, whereas the continued use of the worst SUV will continue generating 1 unit. Simply replacing the worst SUVs at 1 unit with a small gas engine car at 1/3 units will save 2/3=0.67 units or 0.29 units more without the higher extra cost of the new technology. The moral of this comparison is that it is far more effective to discourage the continuation and use of the past largest SUVs than to focus on the ephemeral race to infinitely impressive but misleading mpg figures.

Capacity of California and US for total Electric Only Car Conversion

A government study has estimated that the US has 73% of the power generating capacity to switch to all electric cars if charged during 24 hours, or 43% if charged at night (6 pm – 6 am). However, since the nuclear plants produce power steadily, it must mainly be nighttime capacity in coal fired plants. Not only would this mean much more coal mining and transport, but the coal source would ruin the higher equivalent miles per gallon, which at 60 mpg for the US Average is already only a marginal improvement for a large cost over a hybrid vehicle. The same study said that California - Nevada only had the spare capacity for a 23% replacement of gas mileage to electric car mileage for 24 hour charging, or only 15% for nighttime charging. That was based on using the present fleet of cars which only average 25 mpg and including all miles driven. If a replacement to all Chevy Volt cars is considered, the numbers are much better, although it would not replace all gas miles driven with electric miles. If the 8 kWh Chevy Volt batteries are charged every night over an 8 hour period of normally reduced power, each requires a 1kW power capacity. If there are 30 million such vehicles in California, then this would require 30 Gigawatts of night time capacity. This is near the available range, where the state has about 50 Gigawatts maximum capacity, but can run near 25 Gigawatts at night, but probably not for eight hours. However, each car might not need recharging every night.