I just discovered this podcast, I am delighted it was changed to a daily format; makes a lot of sense!

I recall hearing a reference to cost per mile for EV usage. So, this is the start of a thread about whether cost per mile, or Wh per mile is more important to know for EVs. By Wh/mile, I mean the number of Wh of energy that must be delivered to the EV during the charging cycle in order to drive those miles.

My thought is that Wh/mile is more important because this efficiency may be compared directly to the efficiency of other EVs. If we have the desire, we can calculate the cost/mile ourselves depending on the rate we pay. To do so, consumers will need to get more familiar with their electric rates, and that is long overdue. So, the Wh/mile rating is the single key specification for consumers when considering EV efficiency.

On the other hand, if all we know is $/mile, then it is irreverent unless we also know an assumption for the cost per kWh charge. The $/kWh can vary according to time of day, day of week, season, regional zone, and total monthly usage level. It can vary according to what specific rate schedule the consumer is on, and of course it can vary depending on the utility they buy electricity from. Finally, the $/kWh is not static; there is inflation of electric rates too. In the end, these assumptions open up the possibility for suppliers to misrepresent efficiency claims for EVs, and that will stall the industry.

In the case of ICE cars, we use MPG for efficiency.   There is a reason ICE manufacturers are required to quote MPG figures, and not $/mile; it is because the price per gallon varies. People don’t ask the local Toyota dealer how many $/mile the Prius consumes because the dealer would need to make too many assumptions to answer the question. This should also be the case for EVs.  

Some may argue that the whole point of determining efficiency is so that we may compare the EV to the ICE, and the final cost per mile is the way to do it. Actually, I agree that this is true. But the variability of energy cost means that to make the cost per mile comparison, additional assumptions must me made. So, if we are given limited information about the efficiency of the EV, it is best that it be Wh/mile.

Comments?

I would like to know Wh-Mi in both city and highway driving. Zenn 124 wHr-Mi and Prius 200 wHr-Mi. Say 20c KW/hr and 33 miles per gallon. I like cost of gas equilivent.

124 WHr/Mi * 1 KWHr/1000WHr*$0.20/KWhr=

$0.025/Mi

$2.48 per 100 miles, the cost of electricity varies on area. This works out to $0.82 per Gallone.

since most vehicles will be rated WHr per mile

100 WHr/Mi = $1.00 per 100 miles assuming electricity is $0.10 per KW/Hr

I think you should use national averages.

Electricity costs. 2007 $0.0891 KW-Hr. The strangest stat is electricity prices have actually gone down since 1990.

The national light-duty vehicle fuel economy is 20.2 mpg in 2007.

Thus 100 WHr/Mi = $0.89 per 100 miles

= $0.18 Cost per Gallon Eq per 100 WHr/Mi

How would we figure cost per mile when an EV - like mine - is powered by photovoltaics on the roof of my garage? I've paid off my panels, and they're just sitting there powering my house and my car now... for free.

My current cost per mile is $0.

I think it is WAY more significant to talk in terms of miles per kWh, as everybody's energy cost will be different. See above.

Earlier I said gasoline equilivent was $0.18 per gallon for every 100 W-Hr per mile. Cars that average 22 miles per gallon would be getting about 300 W-Hr per mile. Thus, fuel for EVs is about

$0.54 per gallon equilivent.